What she has going in her favor is that Yahoo was mismanaged to an utterly stunning degree before her time. I don't doubt that a more sensible approach is going to do net-good things for them.
But there's just no way she's going to be included in the same chapter of history as the other names given.
I think she's going to go down as the first female major player in the Big Tech realm -- which is not to disparage Meg Whitman or Carly Fiorina, but they haven't achieved the same level of household recognition that Marissa has.
(I'm not really familiar with her goings-on at Google, but everything she's done at Yahoo seems incredibly positive. Like you said, though, that could just be a 'going from awful to merely okay' effect.)
Think about how much you heard about Yahoo in 2011.
Say what you will about Marissa Mayer(I don't know enough about her). But she's making right decision on who her market is and although growth may be slower, I can see a larger number of people retiring that are going to be spending all sorts of time on Yahoo. Since HN is the anti-thesis of the Yahoo demographic, I think there is a more nuanced and emotional approach that Yahoo is taking. But I could be wrong.
edit: Also if anyone from Yahoo read this, please fix the problem with Yahoo mail and Chromium in Linux. My mom is annoyed by being asked to upgrade(Safari or Firefox) and asked to change the theme.
Tumblr will pay for itself? Not a chance they're going to earn a billion dollars off of that, much less turn a healthy profit. As such, at it seems like all Mayer is doing is burning value on a spend $3 get $1 back exchange. Works great in a stock market bubble of course.
/s