It's a tech company with a market cap of $42bn and see was the fourth CEO to take the wheel in 2012. Of course, what happens at Yahoo! is news and that the conclusion that it went downhill during all the board shuffles makes sense. But I'm not sure this proves she is doing great:
Certainly seems to be in a better state than before. That can be credited to Marissa Mayer. A look at the stock price[1] seems to confirm she indeed does a good job.
However, one might argue that's just mean reversion at work: they hit a bad streak followed by a good one. The fact that this shuffle remembered me of the Henrique de Castro mess makes me conclude see is not doing an 'extraordinary good job', just good.
Before Marissa took over Yahoo wasn't even worth mentioning anymore outside of sentences containing the word 'downhill' or equivalent.