However, it's important to note:
* He is not playing himself. While Fleetwood is a real drummer (and a famous one at that, known for his work with Fleetwood Mac), his character in the movie is separate from his real persona.
* It's a brief, uncredited appearance. Fleetwood's role is a small one in the movie. He's only in the outtakes section of the movie, playing the role of a drummer for another band.
So, while Mick Fleetwood plays Ricky, it's a fictional character within the fictional world of Spinal Tap.