Are you claiming that a being must be a moral agent in order to be a moral patient?
Yes, with the possible exception of moral patients with a reasonable likelihood of becoming moral agents in the future.
Is it ok to eat severely mentally disabled humans than?
It is okay to take them off of life support, if the “severely” part is sufficient. Eating humans is bad for other reasons. I also do not approve of feeding cows to other cows, for example. It causes prion disease.
If someone said yes?
Is it okay to eat a test-tube muscle? (probably yes)Is it okay to eat vat-bodies grown without brains? (probably yes)Is it okay to eat vat-bodies grown with a little brain to keep it growing correctly but that never wakes up?(probably yes)