Look, stereotypes exist for a reason, they help us survive. Stereotypes help us see patterns and give insight into future probabilities, evolutionary psychology 101. Now, stereotypes can fail when applied to various individuals and should never be the end of the analysis, but they are informative.
I used to work in a tattoo shop and I had piercings, but would I leave my child at a daycare with a dude that was all tatted up and had a bunch of piercings and a "Cradle of Filth" t-shirt on? Hell no. I am able to make some assumptions about his character, life experiences, and the type of friends he has from his appearance. Could I be wrong and he's really a saint? Sure. Will I play those odds? No.
A guy wearing thugged out clothing is more likely to mug you than a guy wearing a suit (on the street anyhow, on wall st, the inverse is prob true). We make assumptions about how other people look to keep us alive. If you're willing to break cultural norms of appearance, it logically follows that you may also be more likely to break other cultural norms and this makes people uncomfortable and less likely to trust you. Humans evolved to be wary of outcasts, there was often something wrong with them. The majority of humanity trends toward conformity because it equals "safety." And before somebody starts snickering about "sheeple," remeber, without these evolutionary mechanisms, we wouldn't be here.