Yes, even my own family, for the most part, think I am retarded for this reason. A lot of people will assume you are unintelligent if you have a stammer because being able to talk eloquently is really important to most people. Even if people know better than this, they will still pretend to believe it just to make you look foolish if they are wanting to treat you badly. Obviously it is quite upsetting but I also feel quite used to it. I think you have to be a bit resilient to how other people treat you sometimes purely because it is hard to defend yourself against people when they are mocking you precisely for how you talk. I usually just avoid talking to people because I dislike myself over it and it isn’t worthwhile to see their face change. Regardless, there will always be others who will not treat you so harshly over it and you can befriend them instead. Don’t fight losing battles with arseholes who are trying to humiliate you about it.