I believe people should do what they truly want, but there is no time in the year when this is less the case.
And as I said, an illness, mental or another type, can induce unhappiness any time, and certainly one of those times can be at the holidays.
While I don’t think that isolating completely is healthy when you’re already unhappy, you can choose to step back from holiday-themed events and plan others.
I’m a firm believer that you need to reach out to people you trust when you’re feeling unhappy, depressed or in other ways compromised.