Marissa Mayer is not a good CEO, she maybe was a useful engineer at Google, but she is a horrendous CEO at Yahoo! When Yahoo! outbid FB by a few hundred million to buy Tumblr it was clear, there is no plan. But there was no plan from the beginning.
I'll explain. You come to a new company as a CEO, WHAT DO YOU DO? What do you do first? What would YOU do? You know what I would do (as a CEO)? I would immediately run an inventory of what I have in the company, what do I have to work with, who makes money in the company, who does not make money, what investments are out there, what products, services, people, holdings, cash is out there.
I would want a recount and fast.
Then, as a new CEO I would definitely concentrate on those parts of the company that actually make money because those parts have already done the HARD work of figuring out how this company makes money right now.
Mayer didn't pay attention to the content generating part of Yahoo!, which is the part that actually earns them revenues at all, she didn't give a shit that there is a part of the company that brings in over a billion dollars a year. A BILLION dollars a year and she didn't care to figure out how that's done and how to boost it before doing anything else that TAKES money, any new investments can only be done once you understand your cashflow and you know that you can actually withstand the spending that goes into the investment.
Marissa Mayer was not hired as an engineer to build products, she was hired as a CEO, as a director to direct, to create strategy for the company. Yes, that means coming up with product ideas as well, no, it does not mean coding (which is what she ended up doing herself in many cases), that's a waste of time for her. She should be looking at markets and clients and making sure that her current accounts don't fall off the face of the earth, instead she didn't pay any attention to her advertising income (she stood up her largest clients), her content generating income (didn't even notice them apparently).
The only saving grace for Yahoo! was their Alibaba 1Billion USD investment that brought them money and investors, who used Yahoo! to invest into Alibaba indirectly.
As to products, what products? The fucking piece of shit Yahoo! account that I have (from old days) is horrible on Firefox under Ubuntu 11.04 that I still run on my laptop.