Those of us unconvinced by the claims of greatness surrounding the Oscar-touted film Three Billboards Outside Ebbing, Missouri are likely to feel there are any number of reasons why it should go unrewarded. It could be the way Martin McDonagh’s movie sacrifices dramatic consistency in favour of crowd-pleasing stand-offs orchestrated by the avenging angel Mildred Hayes (Frances McDormand), who first mocks the law, and then takes it into her own hands, when her daughter is murdered. Or it might be the amount of character detail that plainly doesn’t fit with the characters (a sheriff name-checking Oscar Wilde, a pair of hicks watching Don’t Look Now) or the self-reflexive dialogue that keeps commenting on its own supposed smartness. Perhaps it is the plenitude of calculated ironies, beginning when Mildred angrily tells her daughter that she hopes she gets raped – right before she gets raped.

But it seems perverse, especially in light of this embarrassment of glitches, to object, as some have done, to Sam Rockwell receiving acting prizes for his work in the movie. (The latest and most high profile of these came his way last weekend when he was named best supporting actor at the Golden Globes; an Oscar nomination, if not the award itself, seems sure to follow.) The grounds for complaint, according to a New York Times article last week, appear to be that his character, Officer Jason Dixon, is a violent bully whose racism goes unpunished on screen. Though a propensity for beating up black suspects is one of his defining characteristics, he doesn’t pay for this in any direct sense within the film.

Well, here’s the bad news. Behaviour like Jason’s often goes unpunished in art as well as in life, and sometimes an audience has to be trusted to work out for itself which attitudes are to be applauded in a movie, and which deserve our disdain. If the books really need to be balanced on our behalf before the end credits roll on every film, then we can say goodbye to the likes of Chinatown and The Godfather for starters.

Not that Three Billboards is in that class. And it certainly doesn’t do itself any favours by presenting such a muddled perspective, rather than an ambiguous or thought-provoking one. It’s to be regretted that most of its African American characters are used as aggrieved faces in the background, or collateral in Mildred’s fight. Her one black pal seems to exist in the film only so that Mildred can be allowed to use the N-word with impunity – because, you know, one of her best friends is black. This alone doesn’t make the picture itself racist, just careless.

Do you want to tell Robert De Niro that he’s giving back his Oscar for Raging Bull, or shall I?

It is also no reason to shun Rockwell himself for his work in the film. If we’re going to do that, then there will be a very long queue of actors waiting to hand over the prizes they’ve won for playing reprehensible people. Daniel Day-Lewis can keep the Oscars he won for My Left Foot and Lincoln, but not for There Will Be Blood, where he was a proper wretch. And do you want to tell Robert De Niro that he’s giving back his Oscar for Raging Bull, or shall I?

Most long-time admirers of the sparky, surprising Rockwell will agree that the awards he’s getting now represent a clear case of Right Actor, Wrong Performance, since his talents were used more interestingly in Confessions of a Dangerous Mind, Moon and Galaxy Quest. But if there is a complaint to be made, it should surely be laid at the feet of McDonagh, who wrote the part, rather than Rockwell, whose responsibilities begin and end with bringing Dixon to life, not mapping out his fate.

It is disappointing to see so much acclaim going the way of a film like Three Billboards, which is full of fuzzy, diffuse anger that stands for nothing in particular – not least at a time when activism and direct action (from Black Lives Matter to #MeToo and Time’s Up) are making such productive inroads into mainstream discourse. But it’s crucial that life and movies don’t get muddled up in our frustration. Art has to be allowed to be messy and ugly and irritating, and to do something other than reflect back at us our own thoughts and opinions, or our hopes for a perfect world. Anger at real-life injustices should not be directed at fictional characters or the actors who play them, however righteous it makes us feel.

• Ryan Gilbey is film critic of the New Statesman and writes on film for the Guardian, Sunday Times and Sight & Sound