Another nafka mina is that no one considers it a negative reflection on someone if they are blind. No one wants to be blind, but no one thinks badly of blind people and blind people are not embarrassed that they are blind.
On the other hand, it is embarrassing to be stupid, and no one wants to admit that they are stupid.
In general, if people makes fun of an attribute that no one really considers a negative reflection on the people who have it, then it’s not offensive, but if it is an attribute that people really consider to be one that reflects negatively on those that have it, then it is offensive.
For example, I don’t get offended by blonde jokes because I don’t think there is any reason to associate being blonde with being dumb. On the other hand, I do get offended when people make fun of fat people, because people really do think that it’s bad to be fat.