I think it has a very legitimate use, when used as a describing word about actions, instead of a way to mock handicapped people. And I think it's a shame to completely dismiss a word that has a very nice length and prononciation for saying things in anger. It could just as easily be co-opted to mean something different that doesn't mock handicapped people. There's just not a better fitting word for many situations, since "stupid" just doesn't cut it.
Basically, I find it okay to use it when refering to actions like "Elon has done some retarded shit". In such a case it isn't used to mock handicapped people, which is never okay no matter what word you use anyway.