And that's why I say she ran it into the ground- because when Google demonstrated a MUCH MUCH MUCH better Search, Yahoo didn't do anything differently. They kept going as a 'start page portal site', kept pushing their really shitty email system, and just rode that wave into irrelevance.
Yahoo HAD some properties that COULD have been immensely valuable, even post-Google, if done right.
Flickr is one that comes to mind. Google had Picasa, and Google was innovating and pushing updates while Flickr (app and website) stagnated. So with no innovation everybody eventually left Flickr. With some vision, Flickr could have become what Tumblr or Google Photos ended up being.
Yahoo Answers was another one. It was a forum sort of like AskReddit where you could post questions on any subject you wanted and others would answer it. Again, stagnated, became irrelevant.
I'll give you that Mayer came in after much of the value had been lost. But she came into a diamond in the rough, a site that was ready to reinvent itself into something relevant again, and she just didn't.