Why not? Animal welfare is a primary reason why some people don't eat meat. Making them braindead should alleviate the concern for those people, but I have a feeling that the same people would be against it.
I think genetically engineering animals to be braindead is itself the unethical action. If we're willing to sacrifice efficiency we could emulate what animals go through in the wild. They live a life full of socializing and emotions and then slaughtered for meat at the end.
