It's like blaming someone who dropped a last second touchdown pass for losing the game. Yes, it would have been nice if that player could have done a better job. But it would also have been nice if everyone else on the team hadn't made any mistakes either, making a last second play unnecessary.
She was very well compensated for failing, but that doesn't mean that she acted in bad faith. Many executives have compensation packages that are heavily based on performance. Apparently hers wasn't. Ultimately there is a lot of blame to go around for the demise of Yahoo!, not the least to other half dozen CEOs they've had in the last decade or so. You win the fight today based on the preparations and positioning you did years ago. For all her mistakes, that's not on Mayer.