Most of us don't lose all our friends, or go in the deep end alt-right in our 20's...
It seems to me he's reasonably happy, especially considering his circumstances were far from happy before Minecraft - his parents were ex-addicts, he had himself dropped out of school, his father had a mood disorder and relapsed into drug use and killed himself just weeks after the first big Minecon.
No one claimed that it was. Buying a mansion and shitposting on twitter isn't exactly the sign of being a failure, is it?
By all measureable metrics, I'd assume that his life is better than 90% of people that started life in the same socioeconomic class that he did.
I didn't say it does, but the clear fact of the matter is that there is literally no evidence that being rich leads to being miserable.
People saying "money doesn't buy happiness" don't realise that money doesn't bring unhappiness either. You can't use that statement for asserting that some rich person must be unhappy.