Mercy me, a slur? Retarded also has a non-offensive meaning. It just means slowed.
In fact, while I wasn't around at the time I'd wager that "mentally retarded" came into an official usage specifically because it was a clinical, sterile, bloodless, and utterly anodyne descriptive term. Moron, imbecile, and idiot all were once clinical terms. And then people throw them back and forth at each other to call each other stupid, they gained an offensive connotation and new terms were needed.
In 20 years will you find it absurd if people say that "differently abled" is a slur? Will you say "this is nuts, we literally came up with that term to avoid offense?" I will!
Try calling the next Black person you see “negro” or “colored”. The idea that nomenclature can’t evolve is bizarre.
I just showed this to a Black person and they said it was regarded, whatever that means.
More seriously. Is "moronic" okay? It's just an ever so slightly more archaiac way to say retarded. The meaning, the negative connotation, the level of offense—it's synonymous and analogous across the board in its time.
Is it okay because "nobody means it like that, they're just using it as a synonym for stupid?" If so, congratulations, you now understand the other side of the argument better than when you started.