It isn’t even them being total idiots though and I think it’s a very normal response. Lots of stutterers mask so people genuinely stuttering in front of them is very rare.  Lots of friends I have don’t even know I stutter, for example. One friend after years was like “I had no idea you stutter.”  Also, it makes people very uncomfortable and people lose their cool when uncomfortable and do / say things they wouldn’t usually so that’s why they blurt things out. On the other hand, people use comedy to get through uncomfortable situations and these people may even be making jokes thinking it’ll make us feel better (not knowing we stutter) since it provides an opportunity for us to laugh at it and move on.  I think you shouldn’t attribute to malice what you can to incompetence and I think, in most cases, that’s what is happening here.