because the humour was totally inappropriate for almost everything. hell, I actually counted the scenes that were purely serious and I genuinely only found there to be two, of which both are with Heimdall who was pretty much the best character in the movie.
Thor, Loki, and Odin were literally nothing like their previous movie counterparts but complete joke machines. In the previous movies, Odin was serious. Now, he's a senile old man who cracks jokes. In previous movies, Loki was mostly cunning and serious. This one? Yeah, he's been turned into a goddamn punchline of almost every scene he's in. In previous movies, Thor was impulsive, at times serious but mostly joyful? In this one? Yeah, let's turn that shit up a notch and let him become a literal mental handicapped person. It was truly pathetic
Also, a movie about Ragnarok should have been inherently serious to some extent as it was shown in Age of Ultron and not a straight up comedy. Also, Hela really disappointed as a villainess. She could have been great, yet her motivation wasn't properly showcased due to the comedic atmosphere of the movie. I would have had some sort of empathy for her if they hadn't made her a typical bland Disney witch