This is a misleading, bait-y title. Yahoo took a write-down on Tumblr, but that doesn't mean it "lost value" post-acquisition. A far more likely reason for the write-down is that Tumblr was overvalued at acquisition. Paying too much for something is not the same thing as reducing its value.

The title leaves the impression that the way Yahoo has managed Tumblr has harmed the site. It may have, but the article presents no evidence to back that argument up.