Well i'm not sure about the U.S but that's a very offensive term in the UK.
People use that as an insult/slur.
Here we say disabled, or in my brothers case, we say "learning difficulties". If someone were to call my brother mentally challenged though, i'd probably punch them.
what is and isn't offensive varies in the US and Europe so I think it's just one of those things that is offensive in one place but not the other because I've never had anybody have a negative reaction to me saying that
When I used to live in Poland, the school that I went, there was this one boy with learning difficulties would usually make fun of the class because his parents were rich or something and always was looking for a fight.
I remember at school there was this one boy who was annoying as **** and everyone hated him and my friend told me he had severe aspergers and i told everyone to leave him alone cause he was mentally challenged and it turned out he didn't actually have aspergers he was just annoying and i got in trouble for spreading rumours
I have a bottle of headache medicine that I just cannot get open even trying to prop the lid open with a knife doesn't work so my mom's advice was "get a sharp knife and cut through the middle of it"
Yeah the animation is really bad but who can blame them, I mean it was 2002.