Does "mostly right" mean false or basically true?

Mostly right means false. The hypothesis that securities markets are pretty darn efficient, and everybody goes through a broad range of ideas of inefficiencies that turn out not to be "real" (or exploitable) is, I think, virtually uncontested by anyone. Including uncontested by people who think there was a tech bubble in the late 1990s and a housing bubble in the mid 00's.

I hear Fama interviewed after he got the prize. He denies that the internet bubble and the housing bubble were bubbles, in the sense that they were knowable enough to be acted upon. In particular, he claims that anybody who detects the internet bubble and/or the housing bubble will also detect a bunch of non-bubbles such that any action they take to make money off their knowledge of the real bubbles will be (at least) completely negated by what they lose when they are exploiting unreal bubbles.

Efficient Market Hypothesis denies knowable bubbles, at least according to Fama interviewed within the last month.