Tell me you're completely unfamiliar with all of the terrible CEOs Yahoo had before Mayer without telling me you're completely unfamiliar with all of the terrible Yahoo CEOS.
She inherited an absolutely untenable situation and yes, she failed, but given the atrocious track records of the previous five CEOs, she absolutely didn't do any worse and yes, she did at least get the company through the Alibaba IPO so that shareholders got that value.
Let's look at the tenure of her predecessors:
* Jerry Yang: Founder who told Microsoft to fuck off when they offered $45b for a company that was in no way worth that. At least Microsoft didn't get saddled with that shit. Yang also managed to flub the entire Web 2.0 boom despite Yahoo being primely suited for that (and buying early Web 2.0 darlings like Flickr). A year after Yang rebuffed the deal, Yahoo's stock was 60% lower than Microsoft's offer. Truly, this is one of the worst CEO moves in history. Oh, he also turned down a deal to sell the search business for $20b.
* Carol Bartz: Came from Autodesk and had no idea what a consumer web company was like, traded in Yahoo's search stuff (and search deals) for Microsoft's stack (the same Microsoft that had offered to buy Yahoo's search business for $20b just six months earlier) but for very little upfront money and for a deal that was at the high level, only valued at $500m in revenue a year over ten years (which if you do the math, is half of what Microsoft had offered for the search business six months earlier). Didn't understand consumer products at all and let Mail, Flickr and the other acquisitions languish.
* Tim Morse - CFO who served as interim CEO while they found the next replacement. He tried unsuccessfully to sell Yahoo's ad business to Google or Microsoft.
* Scott Thompson - This joker was the OG George Santos, who got caught lying about his education on his resume (which, bro, you're trying to be CEO of a multi-billion dollar company, don't do this) and was fired in one of the most humiliating ways that I can ever recall seeing. He lasted 5 months and didn't achieve anything but making an embarrassment of himself and laying off 15% of the staff. Oh, and he sued Facebook over that ridiculous patent thing.
* Ross Levinsohn - The interim CEO who REALLY wanted the job for keeps but was just keeping the seat warm while the board did the executive search. He didn't do anything wrong per se, but there was a reason he wasn't chosen as CEO. He was a media guy, not a tech guy, and wouldn't have known how to handle what was still ostensibly a tech company. He later went on to lead a bunch of very bad (from a business perspective) media companies, most notably tronc. He's now head of a company that buys once-proud media brands that it licenses out for every dollar it can, killing the legacy and value of those brands along the way.
That's who you were looking at before. Was Marissa successful? Absolutely not. Yahoo spent a lot of money and made a lot of big swings but ultimately came up short. But at least she tried. At least she tried to invest in things like email, search, and content, areas that had the previous five leaders (I guess four is more fair, Ross didn't have a chance to do anything) focused on, the company might not have been in that position.
She also managed to keep things afloat and working so that the Alibaba IPO and Yahoo's stake in that could come to fruition. Given the caliber of the people who led the company before that and the shareholder activism and lawsuits happening before and during her tenure, she did probably as good as anyone in that situation could do.
The bottom line is that Yahoo was not a company that could be saved and she took on an impossible mission, but at least she tried. Yahoo's death warrant was signed in 2008 when Jerry Yang told Steve Ballmer to go fuck himself because he didn't consider $45b a high enough valuation for Yahoo (even tho the price offered per share was a significant premium over Yahoo at the time), mere weeks before Bear Stearns collapsed and the global financial crisis took hold (a position he continued to stick to for months, even when it was clear that the global economy in general was in big trouble).