Perhaps the survivability of a retarded animal might depend on the social skills of its cohorts, if it has cohorts at all.
I can imagine that a hypothetical retarded dog might survive, because dog's have a habit of caring for pack members. I can imagine that a retarded cat can survive, but maybe not as well. House cats, when in the wild, only sometimes stay in prides, but not as tightly as dogs stay in packs. Cats, by their nature are more individualistic, and house cats very frequently survive on their own, only comingliing when it is time to mate or fight. I would think that a poor hunter or food scavenger would die.
I have seen a retarded cat who was a friend's pet. It was very friendly. It never remembered who it met already, so everyone was a NEW FRIEND all the time!! Yayyy!
But, the poor thing could also get trapped in a corner. It would only see walls, and never know to turn around. My friend thought it was a game the cat played, until the cat stayed in a corner for a good portion of a day, and even went to the bathroom there, getting more scared as time went on. So, every once in a while, he had to go "save it" when it got trapped. The vet told him, though, that the poor thing probably never remembered getting trapped, so it only suffered stress when it could not see a way out, but not afterwards.