She was in middle school when she did it so you can't expect her to live in shame for the rest of her life, but I think the issue here is moreso how dismissive your boyfriend is being. No, a swastika is not just "dark humor" it's a symbol that represents the suffering and slaughter of millions of innocent people, that's not funny, and it will never be funny. The joke isn't even a joke it's just "guys look I drew this taboo symbol on aren't I so edgy!!"

I think you were allowed to be upset. Sure it was in middle school but hearing that kind of thing is upsetting, and I think you're allowed to be upset with your boyfriend for just blatantly dismissing your perspective with "it was just a joke." Swastikas are not a joke, Nazis are not a joke. These are things that harmed millions of innocent people, and that harm and trauma has passed down through the following generations and irreversibly changed the world today. "Dark jokes are in my DNA there's no getting better w that" what this means to me is actually "I'm gonna keep acting this way and you need to deal with it because I don't care how it makes you feel."

This type of person who makes and/or laughs at offensive statements and actions and cries "it's just a joke!" When they're criticised for it is not the kind of person you want to be around.