As a dx, not exactly, although 'mental retardation' can be a co-occurence or end-result of plenty of things, genetic cockups usually. Those sorts of insults are low though, plenty of people need insulting, if not most of them. But those sort of terms, insult other people who don't deserve it, regardless of the recipient being a fuckup or not, by virtue of them BEING used as insults.
'spastic' used to be used as a dx, hence the existence of what used to be named the spastic society, as a charity for the mentally disabled+those with cerebral palsy, but thankfully it eventually got renamed and the label removed from use. Never heard the term being used in the US, or as a putdown there, indeed the only people I know from the US who know what it meant, have been a few autistic rights activists who network a fair bit with those of us from the UK.