It's a bit of a reach tbh. "jopa" (жопа) would be a crude way to say "ass". Then some schoolyard nicknames would sometimes have a suffix -lin or -in (-лин,-ин) as general creative addition to create nickname. Unless I'm clueless about of some slang usage, or grandparent refered to some other Slavic language (not russian), "Joplin" (жоплин) doesn't mean anything directly.