That all depends on what you regard to be stupid reasoning, which is yet another matter of opinion. I agree that morals based on stupid reasoning (eg. I think that since I can do something, I must be supposed to do something!) are dumb. However if their reasoning is basically down to you having differing beliefs (eg. Religion-based morals) then their reasoning isn't stupid, it's just something you personally disagree with. You might personally believe it's stupid but that's your opinion, not a fact, and you shouldn't call someone dumb because of it. If anything, it only makes you look 'dumb'.