Bro a "your black certification ran out" joke is miles different than a "white to black trans" "joke". Aside from being inappropriate, it's also just unfunny, and so wildly out of left field, it screams "I'm doing a bigotry reference to be edgy/for attention". It deserves downvotes for those reasons alone.
I don't disagree with your point that Lemmy in it's current state isn't the most casual/normie friendly place. But if you think trans "jokes" are what will make Lemmy more approachable, you and I have very different casual audiences in mind.