To mind flayer all human brains are filled with madness.
And while you can play it as you want, the mind flayers don't feed on "thoughts" but on gray matter, so it isn't important if the mind is diseased so far as the brain is functioning.
But yes, as for
taste the experienced mind and the smart guys taste better. Orcs taste bad, humans taste better, adventurers taste good and adventuring wizards are a delicasy. Dominated people taste far worse than free thinking.
That's why the mind flayers prefer to keep creatures captured in pits instead of just dominating them and then eating them.
Since madness in a way . . . frees the mind and gives strange experiences (like walls talking to you or 20 different minds in a brain) the brain of a madman
tastes better and has a more exotic and rare taste.
PS Info on how a brain tastes for MFlayers can be found in Lords of Darkness only. The Illithiad was lacking in this so important information.