As Barry Ritholz comments:
Yahoo! Inc.’s total value is represented by the first bar. If you subtract the value of Alibaba Group Holding Ltd. and Yahoo! Japan Corp. you are left with Yahoo's core business -- excluding its Japanese and Chinese investments -- and a negative valuation.
I can remember a few examples of the efficient, all-knowing markets getting the valuation of a company completely wrong. It doesn’t happen often, but it's kind of amusing when it does.
Maybe Marissa Mayer isn't doing a terrible job, but Mr. Market simply made a mistake. This chart should make you question whether we give markets too much credit for being efficient and intelligent.