I don't know that they started out humorless--from the first Avengers movie to Ultron, Hulk and Thor always had funny little rivalry happening. The humor was cranked to 11 in Ragnarok, but the Thor movies needed a boost, and these two characters, having been left out of Civil War, got some quality time. My only real critique of the movie is that, to me, Hela felt like subplot--I wish we could've seen her do more damage. Plus, I always want more Cate Blanchett.