Frankly, she did what she was supposed to do: sold the company, satisfied Carl Icahn, and improved internal morale. From what I heard anecdotally, she took an internal culture of rot and at least made it a place where people didn't dread going into work every morning. It's kinda like she pulled the teams out of the swamp, and rested them on a muddy, cold bank of the river. At least they weren't drowning anymore.
The pivot to mobile-first was a big deal, internally. They really lived and breathed that change, as is evidenced by the tremendous amount of engineering they put into simple things like their mobile weather app. It didn't ultimately work, but it was the right direction to be going, honestly.
Was it all misguided and doomed from the start? Sure! Yahoo! was the new AOL, and it was a miracle the company didn't get sold off to a media firm. But then, Verizon is kind of building its own media arm, and it already owns AOL. The fact that anyone bought Yahoo is a testament to Marissa's leadership. Would you have bought it, given the opportunity? I probably wouldn't have either. But she made the sale. If nothing else, that golden parachute is a commission just for ending the damn place as a going concern.
One of the things that really killed Yahoo, that no one talks about, is that they had these security people called Paranoids. They were great, super smart, always available, BUT, they held sway over everything and it was very very easy to just say, "naw, forget that 6 months of work, throw it all away, the Paranoids say we can't put it on production machines."
This was evidently an entirely acceptable thing to happen, internally, and it's why so many things were killed. It's also why you don't hear about constant data breaches at Yahoo! anymore, though.