If you look at the literature on personal interactions, e.g. in a management context, you will find that the general rule is that "you are stupid" is a no-no, but "that is a stupid thing that you have just done" is OK. Sometimes people do do stupid things, and one has to refer to it, and say, yes, that is stupid. But one can do a stupid thing and then recover from it, whereas if you say "you are stupid" that implies a permanent problem.