In the same story we had Joker busting in on two unknown elder people living in their apartment and brutally killing them just because. That wasn't exactly creative or "joking", yet I don't see it is a point of critique here.
The creativity of the rape was actually fitting when you look at it from the point of context, how it related to the relationship between the Joker and his biggest fan and what had led to the wife beind in that car in the first place. It was a price, it was a cruel emotional deed, and that was what the Joker was about in the story.