I dunno, but don't wear big earrings. I wore my 3/4 inch vintage red rose earrings to a Gaslight Anthem show and their opener was Against Me! and I almost had one of my earrings knocked out. (I hate hardcore, lol)
I'm sure than I can be of some help here. Since I listen to a ton of 'hardcore' bands and attend a few concerts, I'd say wear feminine black clothes. I wouldn't say that most guys that like 'hardcore' music are into 'girly' gals, but rather ones that wear dark versions of regular clothes (an example being an off the shoulder black shirt + skirt & Doc martens). 'Shortish skater type skirts can 'accidentally' get flipped in a mosh pit or a dancing crowd (my friends know from experience). Jeans would certainly save you from getting scraped or scratched (another common mosh pit occurrence next to bruises) So it depends if mosh pits do occur, but you can't go wrong with jeans, docs, a crop top or tank top with an overshirt or cardigan on top.