I see everyone say John is depressed in the credits but I don't see it, could you point it out to me?

A big part of the Characters leading up to the ending, is their Nostalgia for simpler times, and their Expectations of what comes next. Kanaya knew she would be raising the next generations of Trolls, and Rose tagged along. Roxy has Callie with her and Janey to keep her company and chill as the Queen of the Carapace Kingdom. Karkat and Dave are together and go out on adventures with Jade, who may or may not also be together with them- This ties into Dave’s interest on digging up dead things, except instead he’s digging up liberties. Dirk and Jake do robotics together and spar and may also be together- On top of that, Jake has Skaianet, just as Jane has Crockercorp. Everyone has someone or a group of people they hang out with regularly, do stuff together, or at the very least, have some job to look forward to.

John, however, never expressed wanting anything after the Game was done. His biggest wish was for to fight until the end and make sure his friends were alright and… He got that wish. Even Terezi, who didn’t get a happy ending, is at least actively looking back for Vriska, searching for her for Years in the Collapsing Remnants of Paradox Space. She’s driven with a purpose.

John is not. He has nothing to do. He hasn’t expressed any wishes of what to do next. His actual Dad is gone, as much as Jane’s may look like him the Credits tells us he misses HIS Dad. He doesn’t do any King-like stuff in the Human Kingdom, in his house, without the drive to even get up to take down the Birthday Decoration from two years ago.

By the end of the comic, the Protagonist has finally gotten past the story- And so now, his purpose, which was to reach the end of it, has been fulfilled, and he’s left not knowing what to do next.