At least as of July 2013, the appreciation in the stock price was due to the increase in the value of Alibaba and Yahoo Japan.
"But Yahoo’s turnaround remains very much a work in progress. Although Yahoo’s stock price has soared by a whopping 73% since Mayer become CEO, that gain is almost entirely attributable to investments the company has in two Asian companies, the Chinese e-commerce giant Alibaba and Yahoo Japan. Yahoo’s core advertising business remains sluggish amid intense competition from rival Internet giants like Google and Facebook."
Besides, stock price isn't a good measure of de Castro's performance. Advertising revenue is better. Over the past year, Yahoo!'s advertising revenue has declined while its competitors' have grown. Facebook took the #2 spot in US digital advertising revenues (Google is #1) from Yahoo! last year.
If Yahoo!'s advertising revenue had grown increased at the same rate as, say Google's (up 15% over the past year), revenue would have been ~$600m higher.
It's interesting that de Castro started out at McKinsey. The book 'Dangerous Company' portrays McKinsey consultants as Powerpoint jockeys who are great at formulating high-level strategy but not so good at actually running businesses.
Perhaps de Castro benefited from a rising tide at Google but lacked the turnaround skills required at Yahoo!
