LimpingNihilist

It's not just a limit; you can make fun of whatever you want. If anything, you should only consider the audience of the joke before actually making it. For instance, if you know somebody is offended by, say, a slur (or the 'time' or 'place' are wrong, as the adage goes), you should probably try to avoid it. However, if you know that these conditions are not met (or have no reason to believe that they would be), you could make a joke with that slur. However, one does not need to be of the group that is directly affected by that slur to use it.

Words have the power we give them. If one removes the prejudice attached to the word (ie, recognises the context as being a joke and not meant to cause actual harm), then it is easier to laugh at it.

Furthermore, making jokes of this kind fall under two categories. Firstly, they can be a genuine joke meant to degrade that group. Secondly, they can be a genuine joke meant to degrade the group that would make a joke of the first category. Hanlon's Razor suggests that, if we encounter one of these jokes in the wild, we should not presuppose that it is done in malice without reason to believe so; in the meantime, we should presuppose that it is stupidity -- one of the foundations of comedy, usually Nonsense Comedy.