Well, get used to it. Hot Topic used to be a cool store when they started popping up in the late-90's, now they are a corperate flea market that caters to every "dark" genre without being too dark. It's a store that has policy to not sell products with Pentagrams or Black Metal merchandise, and is more concerned with playing nice and not causing an uproar and sells parent-friendly rebellious crap. The Joker fascination, well, that was coming as soon as those first joker shirts sold out. In a couple weeks, you'll see purple trenchcoats and green vests on all the walls there.
As for your "chubby" female friend, that was coming as well. Heath Ledger was a sex symbol before his death. The comic stuff is typical ignorance, I seen the same crap when the Crow was popular. Girls were swooning over Brandon Lee, but most of them had no interest to read the source material. Dont worry, just like poser fanboys, they will move on to whatever is Hot in the next couple months.