Jimmy was accustomed to his friends making fun of him.
This is not a straightforward expression…
The English sentence uses an active form (making fun of), but we usually say this in a passive form (being made fun of) in Japanese.
My alternative translations are:
へっちゃら means “I’m totally okay with that, and don’t care of that”. Inserting いつも (always), making fun of him doesn’t mean a one-time action but repeated actions. So, you need いつも followed by へっちゃら if you mean “be accustomed to (a bad thing)”.