A better metaphor might be: Yahoo! was like an old rusted out car. You hire someone who tells you how they can fix this car, you pay them a lot of money, and then they leave the car in worse shape than when you hired them. How much blame do they deserve?
Which is to say: A new CEO absolutely can turn a failing business around -- that's what she was being paid tons and tons of money to do. If Meyer did feel like Yahoo! was a completely lost cause or that she would be incapable of fixing its problems, then she should not have taken their money. As an extremely wealthy person to begin with, she had that luxury.
In short: She absolutely has culpability in the failure of Yahoo!.