Dave Mustain is the very talented Guitarist and lead singer of the heavy metal band Megadeth, he is otherwise notorious for getting kicked out of the band Metallica in his early days because of his unhealthy behaviour. Among amateur rock musicians, likening a band member to dave Mustaine means that they are unprofessional and that their general attitude impedes on the overall quality of the band, because they can't walk the line for example. Note that the real Dave Mustaine is a musical genius (regardless of his other problems), but a "Dave Mustaine" doesn't have to be particularly talented.
-Hey Slash, how's your band?
-It's O.K., but our lead singer is a fucking Dave Mustaine! We probably wouldn't keep him if he wasn't so talented.
-Oh, he is good? So he is like a real dave Mustaine then!