I get what you're saying. But I also don't think it is as clear cut as what you're making it out to be. There has been cultural acceptance for things that shouldn't be acceptable at. That's still the case now. The nuances for jokes in this role, I'm sure are subject for more than one doctoral dissertation.

I also stand a little bit corrected, because I don't mean that all jokes are OK. I think however that all subjects, are. When the premise of the punchline depends on the controversial subject being OK, it fails at doing so, and I would agree with you.

For example, there is a huge difference between a joke that is funny because it predicates and reinforces a stereotype that "black people are X", vs a joke that makes fun of the stereotype itself that "black people are X". The latter is OK. Doesn't mean everyone should find it funny. But it can certainly serve to ridicule the stereotype itself.

Which brings me to this:

No rape joke is funny, and harms rape victims by making light of their suffering.

Why do you think all of them "makes light of their suffering"? I would argue that it depends entirely on the joke, the context, and the punchline itself. The joke in question in the comic here, does no such thing. The joke is the classic "set up a expectation => subvert it", and what it does, if anything, is bring attention to "rape happens", and "rapists exist". Awareness of that, is a good thing, no? Isn't it more harmful to make rape tabu, almost as if pretending it doesn't happen, and that rapists don't exist?

Which is why I'd like to argue the exact opposite of what you suggest. And this to say that comedy is an extremely important mechanism, if used right, to bring attention to problems that should be talked about. Especially uncomfortable subjects. It can educate, bring awareness, and challenge stereotypes or established societal constructs. All subjects are therefore OK. It doesn't mean all jokes are funny.

Tl;dr: context matters.