I can't understand it for you, I can only explain it to you. Even then, I don't know if I have the time, patience, or crayons to do that. You see, "mature" "humor," which is two lies in one in Hazbin's case, relies on shock value, socially frowned-upon subjects, and crudity. Generally speaking, it's hilarious on first watch, and nothing but comatose crickets on the second. Actual mature humor, like King of the Hill, relies on wit, relatability, and subtlety, with just enough absurdity to make it funny, but just little enough for it to still believable. Bojack Horseman also accomplished this decently well, assuming you're willing to ignore the fact that most of the characters are furries.