thick, dense, dim, slow, brainless. they're all equally pretty rude, just by the fact they're grading someone's intelligence as being low. They could be used to describe someone who has geniune low mental ability, bordering on mental disability but I would never describe anyone in this way as it sounds insulting and would probably be taken as such, even if the person had been medically assessed to have lower abilities. Amongst friends who are giving each other friendly abuse for their behaviour, then they're all ok as long as you are joking and you know it will be taken as a joke.