Mind flayers are, of course, underground-dwellers, living in the underdark, far away from the sun. Entirely theoretically, though, say a mind flayer was above ground for an extended period of time - like, he abandoned his colony and set up a shop or something. Would long-term exposure to the sun damage the mind flayer's skin? I'm pretty sure that they secrete a protective mucus to keep their skin from being damaged by harsh conditions, or something like that? I mean, I know that many subterranean creatures can be damaged by extended exposure to sunlight. Let's say the mind flayer managed to obtain a steady source of humanoid brains, and as such at least wouldn't starve to death. Would exposure to sunlight shorten its life expectancy or something like that? If so, could it survive longer if it stayed inside a dark building?
In the Monster Manual (5th Edition) there is nothing talking about sunlight sensitivity or any other weaknesses related to sunlight.