For a long time now at least here in the US it is quite frowned upon.
Say whatever you want, just saying it makes me and the vast majority of people in the real world think of a slur for folks with disabilities
Discussion
I think this is probably true for the bubble you live in (I don't mean that pejoratively), but that for most people it's just a synonym for stupid.
It's really not used in my bubble either, but I recognize that I don't regularly interact with a representative sample of the country.
US isnt vast majority nor real world.
Why is that word not okay but "stupid" or "idiot" is ok? Thats not making fun of mentally retarded?
Double standards
If I say "you are mentally slow" is it ok?