The thing that complicates it for me is that laughing isn't voluntary. If you make an offensive joke that's really funny, I'm going to laugh. So yes, rape jokes can be funny. That doesn't mean it's ok to tell rape jokes, I don't know. But they can definitely be funny.
Making jokes about something bad is a coping mechanism. I had a medical situation earlier this year which for me was very saddening and still is, and they way I helped get through it was to make jokes about it. People around me weren't sure if it was ok to laugh so it became interesting to observe their behaviour.