You can joke about anything and you can say pretty much anything, there's a difference between what you can say and what you should say in my opinion. You can make a racist, transphobic, or otherwise terrible joke, I'm not saying you can't, but I'll probably think a little less of you for doing so (not that anyone cares though). Anyone can say anything, you'd just think morality would play a part in what people would feel comfortable joking about. I personally like jokes about dark/morbid subjects, but jokes at the expense of certain groups of people just aren't funny to me.
Tbh I love telling and hearing dark jokes, but what I would and wouldn't say depends on the audience.
People can makes jokes about anything and I won't get offended if they don't mean it. However there are stuff that I won't find funny IE jokes about incest with my family. I laugh at racist jokes including my own race.