Why would anyone want to?
Especially since it's materially dangerous to eat people. Cannibalism risks a lethal debilitating prion disease, which is why when we fed cows to cows we got "Mad Cow Disease."
That aside, I said rights are attenuated to abilities and potentials, so for your question even to be relevant you must mean someone who is not merely retarded but is nearly brain dead (such as with the permanent cognitive capacity of a baby). I also said rights are as we create them. These are simply true facts of the world.
So assuming we encountered anyone who for some strange reason wanted to eat, let's say, brain dead patients in an ICU, would we want to permit them? We already cannibalize brain dead patients by removing their organs and putting them inside other people's bodies, which is directly analogous to digesting them. So if it were medically safe (though it is not), there would be no evil in allowing the rest of them to be consumed in any other way. Otherwise those bodies are just treated like garbage (which is what burial or cremation amount to). There is no rational argument that the one is more dignified than the other.
However, the rights of a brain dead patient are not the only ones of concern. We also create rights of their relatives or even the state to decide what happens to them (as this affects the pursuit of happiness of the living). That's why we don't just cannibalize every brain dead patient for transplant organs. Their legal guardian must permit this, and if they don't, we respect the rights of that guardian, who by definition is neither dead nor retarded.
So when we turn our query to patients who are not quite brain dead, but massively and unrecoverably retarded, first of concern are the rights of their guardian, who will either be an immediate relative or the people (represented democratically by the state). So the question must be, assuming for some really strange reason those guardians wanted to humanely kill and then eat the retarded person, should we let them? The question is directly analogous to asking if they should be cannibalized for transplant organs.
We should not allow this not only because it encourages people to dehumanize others (which will have the unwanted collateral effect of also causing more callous treatment of everyone, not just the mentally retarded), but also for the same reason that we should not allow the execution of criminals (apart from genuine cases of self defense): it is not necessary, and thus needlessly risks killing an innocent. Our ability to ascertain the degree of someone's retardation is not 100% reliable (as indeed was recently proved in court battles over the morality and legality of executing retarded criminals--as Texas in fact did, which is hardly any better than eating them, and arguably worse), nor is our knowledge of potential future treatments and cures for retardation 100% reliable. Thus considering whether we ourselves were found to be in such a state, would we want to be killed, or kept alive and well in the event that we may one day be cured or were never as retarded as our doctors thought we were?
The answer is uniform and obvious. We legislate accordingly. If only we acted as sanely in the matter of capital punishment.