I believe that, to a certain degree, the humor in most really great comedies tends to be on-the-nose with respect to a particular context, which often means being time-specific. In the case of 'This Is Spinal Tap' (1984), if you were attending Black Sabbath and Judas Priest concerts in the early 1980s, the film is much funnier than if you were not. (To an extent, in my own opinion, I think the same principle applies to 'Annie Hall' (1977), as I found 'High Fidelity' (2000) - a similarly structured/themed film directed toward my generation - to be much funnier.)
Of the four related mocumentaries from roughly the same group of principles, there always seems to be at least one that falls flat - depending on the viewer. For me, it was 'A Mighty Wind' (2003) - the mocumentary on the self-important folk music veterans; for my spouse, it was 'Waiting for Guffman' (1996) - the mockumentary on self-important small towns/community theatre groups.