I feel like an idjut, but what did Sam mean by "mullet rock"?

it's definitely not what it's called, it was a joke ! :D Sam is saying Dean's music is the kind that is made by and listened to people with mullets, generally white trash. Picture Ash, and imagine what kind of music he would make or listen to ! :)

He means that all of Dean's music collection is like 80s head-banger rock music:D

He basically meant that all of Deans music was from the early 80's which was the time of the mullet hairstyle and rock music.