I suppose they are cameos in some ways but people like Fran Descher weren't really that well known when Spinal Tap came out so she was more just an extra I think. But Rob Reiner is in it, Dana Carvey, Billy Crystal, Paul Shaffer, Angelica Houston, even Blackie Lawless got a scene or two.
Isn't every hip hop movie a comedy of itself? I know a few years ago Ice Cube put his son in the role of what was essentially the bio of his coming into fame. I'm not sure how comedic it was though I think like most movies about black youth it was probably less comedy more 'look how rough it was for us'.