First of all, if indeed one is mentally deficient, that is not an insult. That is a statement of fact. It is something that person and those who live and work with him or her must deal with. It makes no sense to use an inaccurate label to describe someone whose opinion you disagree with.
On the original Star Trek series, one of the characters was a half-Vulcan half-human. As a result, his blood was green. When one of the other characters on the show tried to insult him by calling him a green-blooded half-breed, he just shrugged. Why be insulted with a statement of fact?
But if one is not mentally deficient, and is so-labelled, that is just plain illogical. Again, it is not an insult because it has no truth. But the charge throws the discussion completely off track. It is the speaker's attempt to avoid the discussion by claiming that his or her opponent is not worthy of debate.
It is also an admission that the speaker has on solid arguments on his or her side and therefore has resorted to attempted insults.
My appeal? If you are going to debate, do so. If you run out of arguments, politely table the topic. But don't corrupt the English language just because you can't think of anything sensible to say.