What makes you say John is depressed?
Every time John has nothing to do he dwells into a spiral of detachment and loneliness. In the 3 year trip, he ended up rejecting Nic Cage’s movies and pushing Davesprite away, something he later regretted. After the game is done, he has literally nothing left to do! The game is won, his friends are safe. He never expressed wanting to do something in the future, like, every other character had some sort of goal or purpose beyond SBURB, but John was never interested in anything he was particularly good at, nor did he seem to want anything else than for everyone to get together and be happy.
So once he gets his wish? He just. Sits there, at home. Occasionally chatting with his friends, too dysfunctional to even take down the ‘happy birthday’ sign for two whole years, remembering his Dad nostalgically. He’s just a Kid who’s also a God and where everyone else got married or has a company or some sort of bigger hobby, he just. Doesn’t have anything. Ironically fitting. He’s the Protagonist of the Story. Now the Story is over, and he has no Story left to go through. So he doesn’t do anything else. And it slowly consumes him.