Yeah, the ex-Googler's I've met have always been average; but I guess if no one will say the emperor has no clothes it doesn't matter
The frame of reference here is important, otherwise average is pretty meaningless since we have no objective list of what an average developer consists of.
As a hiring manager I view Google on someone's resume as a non-signal. If however they are the sort of person who draws attention to having worked at Google I view that as a negative signal.