Let’s do some basic math about Yahoo since Marissa Mayer took the helm over three years ago.

She paid about $3 billion for acquisitions of companies you’ve mostly never heard of, like Aviate, Polyvore and Distill (and one company you may have heard of, Tumblr). She spent $9.4 billion on stock buybacks; over the last two years, when the stock was trading higher, the buybacks have been a $2.5 billion money-losing trade. About $365 million of compensation went to Ms. Mayer herself, assuming she stays for an additional year and a half. And $109 million to an executive she hired to be her chief operating officer, who was then summarily fired 15 months later. An estimated $450 million on free food for the staff. And, depending on whom you believe, double-digit millions of dollars on parties and events, including a “Great Gatsby”-themed holiday party several weeks ago that was held with no apparent irony.

Many of those figures come from a devastating new presentation sent to Yahoo’s board over the weekend by Eric Jackson, who runs a small hedge fund called SpringOwl Asset Management and who has long railed about the company’s missteps.

Inside Yahoo, Mr. Jackson is dismissed as a “small-time” shareholder who “doesn’t have his numbers right,” according to one person involved in the company’s recent travails.

But whatever the size of his stock position in Yahoo or even if his numbers are slightly off, there is an underlying truth to Mr. Jackson’s critique: Despite Ms. Mayer’s insistence that Yahoo is a very different and better company than the one she took over three years ago — and it may well be — it is being valued as if she has done not much of anything.