Yahoo was; iirc a negative asset on the balance sheet and Japan and Alibaba were only drivers of actual value. Given how much Yahoo had to pay to entice her, in hindsight paying $1.00 for a carrot at stop and shop to be CEO of Yahoo would've had the same result until they went into sales negotiations
Before she got there, Carol Bartz was the chief. She had royally FUCKED UP the relationship with Alibaba. As a result, BABA refused to go public until it got back the shares from Yahoo; and, in fact, Jack Ma took Alipay out of the consortium in a fit of anger, without even consulting Bartz.
In short: the relationship was in dire straits.
Mayer comes in, smoothes things over, strokes Ma's ego and next thing you know, Yahoo gets to keep most of the shares, BABA goes public and Yahoo's value skyrockets. Mayer had absolutely everything to do with this.
Edit: for more reading: https://techcrunch.com/2010/09/12/bartz-in-a-china-shop-has-...