One possible reason is that she is a woman. This, seemingly irrelevant fact, has a couple of potential implications.
First, criticism towards one of the few female tech CEOs could be viewed as misogynistic. Most people would shy away from such controversy.
Second, as a woman in tech she is the underdog, and people love to root for the underdog.
Just a theory, I could be way off of course.
(In particular, I remember a lot of this sort of anti-Yahoo sentiment in the months prior to Marissa Mayer, particularly around the time that the Yammer CEO made some announcement that he would not be hiring ex-Yahoo employees because of Yahoo's patent stance. People decried that CEO as being ridiculous, but there were also a lot of people expressing anti-Yahoo sentiments.)
Probably some combination of both, as well as a few other factors.
Her CV begs to differ: she was VP of Search Products and User Experience, and then held key roles in Google Search, Google Images, Google News, Google Maps, Google Books, Google Product Search, Google Toolbar, iGoogle, and Gmail.
That said, you're very wrong. She had a defining impact on Google Search from the start, a project that did in fact produce notable success and revenue for the company.