Not generally. Self-imposed isolation, because you have the power to end it, doesn't damage in the way that absolute isolation imposed by others, may have. Learning is good for the brain, and inded there is evidence that in the late phase of life, people who continue learning and chal;lenging their mind, remain more mentally alert and efficient than those who just laze around and watch soap operas.
Anyone might have mental "issues", and perhaps iof one isolates oneself it is easier to focus on them unrealistically and see them as worse than they need be.
It can be hard to distinguish cause and effect - is someone unhappy because they socially isolated themselves ? Or did they isolate themselves because they began to feel unhappy and uneasy in public ? A good assessment by a good shrink can help clarify whether there is anything significant wrong, and if so, how best to remedy it