Who could fail to be amused by the spectacle of a besuited mayor of London stuck dangling halfway down a zipwire above a public event in Victoria Park, east London? It was certainly an original way for a politician to hog the headlines. So was I the only seasoned Boris-watcher to ponder, if only for a moment, as to whether it was all part of the latest ingenious Johnsonian masterplan to win hearts, lift moods and go global on his cult status? In other words, was it the next bizarre but calculated step in his tactical assault on Downing Street?

The incident was certainly a public-relations bonanza by the master of political daredevilry. It left David Cameron noting, with what sounded like envious resignation, how Boris Johnson could turn what would be a disaster for any other living politician into an "absolute triumph". It widened the gap between Johnson's invigorating brightness and Cameron's pessimistic realism; the blond's opportunistic genius and the brunette's apparent lack of ideas. It cemented Johnson's status as a towering national – even international – figure and the man who owns the London 2012 Olympics (or at least its successes).

Recent polling has suggested his box office status would not only secure him the party leadership in a future contest, but even a fighting chance of beating Labour at a general election. In fact, Johnson is so spectacularly popular he's probably the only politician in the country right now who could exult in what he called his Murdoch "schmoozathon" at the Olympics and get away with it. (By contrast, when Jeremy Hunt was caught quietly shaking hands with Rupert Murdoch last week he looked understandably furtive.)

Consequently, many in and outside the Conservative party have started talking about "when" and "how" Johnson will become prime minister, rather than "if". The startling omission in all this outburst of Boris-mania, however, is whether he "should" be considered as a prime minister at all. Before we get too carried away by the cult of personality, it may be worth glancing at his track record. Any whiff of scepticism – well-founded or not – over the zipwire show should be judged in the context of Johnson's extraordinary record as a performer. Those wonderfully spontaneous bumbling speeches, such as the Conservative party conference one that so baffled Arnold Schwarzenegger, are meticulously planned. Former staff reveal how the pauses, the non sequiturs, the rambling tangents are studiously prepared; the most successful jokes and "off-the-cuff" Boris-isms are rehearsed and recycled.

He perfected his act on Have I Got News for You, discovering how a display of stage incompetence endeared him to the audience. When his phone went off on air, his popularity soared even more. Only a handful of cynics who had worked closely with him questioned whether the timing was purely coincidental. Only when the joke wore off after several other similar occasions, including in the chamber of the House of Commons, did the phone mysteriously stop ringing. But does a flair for the comic turn count as a qualification for national leadership in a crisis? Does raising a few laughs produce economic growth?

The zipwire show was also a brilliant showcase for the trademark Johnsonian bonhomie. As the news cameras rolled, he beamed down conspiratorially on the growing crowd beneath him and they loved him for it. He has this gift, sometimes enhanced by looking directly into the camera, of appearing to address each of his audience in person. Viewers are rewarded with a sense of ownership of the mayor of London that owes far more to celebrity than politics. It is a potent talent. During his mayoral election roadshow in May, it was extraordinary how, rather than running away, people wanted to kiss him and touch him. Some did not want to let him go. And this is a politician we're talking about!

And yet for all his apparent friendliness, Johnson is rarely a friend. In fact, although many might describe themselves as a pal, they are usually mistaken. As a critic once observed, as with Lord Palmerston, Johnson "does not have friends, merely interests". Indeed, when questioned, these self-professed "friends" often admit that they have seen the mayor socially perhaps only a couple of times in the past few years. Those who are no longer "useful" have not seen him at all.

Most admit they have rarely if ever conducted a lengthy conversation with Johnson; he is not one to share a pint at the pub or a club with a mate, for instance, and also only likes to run alone. One former female aide recalls how she dreaded car journeys with him as conversation would either be painfully stilted or simply non-existent. At gatherings, it has been his habit to avoid "one to ones" and escape the embarrassing intimacy of such encounters by constantly introducing people to someone else. Even at private dinner parties, senior Tories say he will offer to make a speech to avoid the agony of cosy two-way chats at the table and the possibility of direct questions. He prefers to be in "transmit" mode rather than "receive". It is as if he erects the highest walls around himself to avoid any of us really getting to know him. What does he fear we would find out if we did?

Nor does he show loyalty to those who have helped him most but are no longer in a position to do more. Those who went out of their way to propel him to stardom, such as Conrad Black, the former newspaper proprietor, and Johnson's former editor and mentor Max Hastings, have received scant return. When Black was facing criminal charges in the US, Johnson ran an attack on his patron's "murky business origins" and "clumsy provincialism". Johnson has also accused Hastings in print of "rank cowardice" for not following him on to Have I Got News for You and has failed to pay up on a lost bet over the result of the 2010 general election. Hastings, who has known him for nearly 30 years, still has affection for his former protege but has also sounded warnings about his unsuitability to become PM, not least because of his "startling flashes of instability". To those who have worked closely with Johnson, his outbursts of temper are notorious; even his sister, Rachel, describes his approach to those who dare to criticise him as "Sicilian". Female members of the London Assembly have lodged a formal complaint about his offensive conduct.

In politics, Johnson has long been distrusted by many, even in his party. He successively conquered Eton, Oxford, the Daily Telegraph and even party conference but in reality he failed in Westminster as an MP, flunking his chances of major promotion by an apparent professional death wish. He made little impact, not least in the chamber where he was dismissed as a lightweight and "echoing parody of himself". He was notorious for avoiding the typical legwork normally expected of a backbench MP and was on occasion, when it suited him, destructively disloyal to his party. Some complain that he continues to shift the blame quickly on to others, such as the Olympic security near-fiasco, when anything goes wrong on what could be called his patch. He is equally quick to scoop the glory for the successes flowing from others' hard work.

It is his good fortune that many members who particularly resented him have now left politics and the new intake judge him simply by his stunning success in regaining Labour London for the Tories. But while his winning credentials are not in doubt, his lack of team spirit could still count against him. It is perhaps also his distance from others that leads him to make such questionable appointments as mayor. His early months in City Hall were marked by a rash of scandals, resignations, sackings and even a criminal conviction among his senior staff. So acute was the chaos and infighting that one old hand felt obliged to take Johnson out to dinner to instruct him in no uncertain terms to "get a grip and start being mayor"! So far, his second term has already been characterised by an astonishingly inept yet arrogant performance by his new deputy mayor for policing. On this basis, can Johnson really be trusted to select a cabinet?

While his stint as mayor has undoubtedly been brilliant for Project Boris, it is far from clear that London has equally benefited. The capital has some of the highest public transport fares in the world, yet offers an unreliable service. Its police force has undergone its worst internal crisis for a generation, with the mayor burning through three commissioners in as many years. The Boris Bike hire scheme, while popular, is a financial swamp costing more than £100m; we are in danger of breaching EU rules on pollution; the cycling death rate is rising; there are disturbing trends in some areas of crime. Does this qualify him to become prime minister in times like these?

Boris Johnson is unparalleled in politics in terms of self-promotion and even occasionally cheering us up. He is hugely clever and politically astute. But after more than four years as mayor, he has yet to prove himself in action, let alone as a contender to be prime minister.

Sonia Purnell is the author of Just Boris: A Tale of Blond Ambition