Here's the thing: If Yahoo was in such dire shape that it couldn't be saved (that seems to be the consensus), it should have been scrapped and sold for parts to maximize shareholder value.
Instead Marissa squandered hundreds of millions of dollars on random acquisitions and Katie Couric. In my mind there is nothing noble or defensible about coming into a dire situation, throwing a Hail Mary and then shrugging when it doesn't work.
The board hired her presumably believing that there was some non-zero chance that she would manage to turn Yahoo around, just as it has done with a string of past CEO's. Presumably they also surmised that even if she didn't, shareholders would come out of it ok.
And lo and behold: When she joined the share price was below $16. It's now above $42. A lot of that (maybe all?) is the Alibaba holding, but the point is that whether not Mayer did a good job or not, Yahoo! as a whole is still worth a lot more today.
But presumably this is part of the reason they took the risk: Yahoo!'s core businesses were valued extremely low - the total value of the business is tied up in the Alibaba holding. As such spending hundreds of millions to try to turn things around meant betting with a miniscule proportion of the value of the company with a potential for huge payoff if it succeeded, and very little downside.
In retrospect its easy to say she didn't achieve what they hoped. But it's not so easy to say the alternatives would've been better.
I'm sure she did stupid acquisitions. I'm also sure most of Yahoo's CEO's have done plenty of stupid acquisitions. I'm equally sure Yahoo's board signed of on all of them.
The point isn't that she has no blame. The point is that she is par for the course for Yahoo, and there's little indication someone else would have achieved better results.
Instead Marissa squandered hundreds of millions of dollars on random acquisitions and Katie Couric. In my mind there is nothing noble or defensible about coming into a dire situation, throwing a Hail Mary and then shrugging when it doesn't work.