He goes over it in the video but I don't think Thor's sadness at failing so many times really hit this guy as much as it should have. He failed to protect Asgard from Hela/Surtur, the people he DID save died with Thanos in the beginning of Infinity War, and then he failed YET AGAIN to kill Thanos and bring his people back. Pretty much everything that happened to Thor since The Dark World has been him failing in one way or another. He was sullen in the beginning of Endgame, and when he finally was able to kill Thanos, it just wasn't worth it, at least not enough to bring his confidence back. He'd been dealing with that pain for so long, when Thanos was gone and there weren't any more major battles to fight, he did what most people probably would in that situation and let himself go. I think it's pretty well done, myself. Sure it would've been nice to see Thor be a bad ass again, but I think it makes total sense for him to be acting the way he was.