Not directly proportional but a lot of times there's a clear link, especially if a person is very dedicated to the music and the particular scene in question.
Hardcore metal heads tend to have long hair, black jeans and band shirts.
Hip-hop fans wear sneakers and baggier clothes.
People into folk music maybe dress a bit more bohemian.
This is hardly surprising and it's very natural. I don't think you have to be shallow at all to strongly identify yourself with a group. I've always been drawn to sub-cultures and I def see a link between my interests and how they connect to each other. Then of course there's usually something that seems "odd" or unexpected in a person.
This is probably true to some degree, but I'm pretty sure there are more people who have such conceptions about people than there are people who actually fullfill these criterias.
It is kind of fun though that people who don't know me always get so surprised when they see that my ipod-library consists of 90% hip-hop and rap