I'll be honest. I had ZERO interest in ever watching the first Joker and still haven't to this day. But when I saw how much people hated this one and looked into why I found two answers "one, it's a musical" and "two, it's plot seems to be a direct attack on the first film's fans". I find both of these things amusingly interesting and it kinda makes me want to check it out one day. Based on that alone I can see this film finding a cult fanbase down the road, so maybe he's right.
Both are untrue as to why people en masse dislike this movie. It's just a supremely dull movie. Nothing interesting happens the entire film, it's basically just regurgitating a question that was already asked and answered in the previous film. The fact that the movie has no thesis of its own is made even clearer by the fact that the movie constantly reminds you of what happened in the first movie. Like, the entire second act of the film basically only consists of characters from the previous movie and new characters telling Arthur and the audience stuff that happened in the first movie.
The songs are also just incredibly dull and pointless. Instead of the songs being vital to the narrative being told (like, you know, a musical) they feel like a tacked on inclusion that's only there because Lady Gaga agreed to be in the movie and you can't have a movie with Lady Gaga where she doesn't sing. Nothing communicated in the songs isn't also simply communicated through dialogue.
It's only in the final 10 minutes of the movie that finally something interesting happens, but it's pretty much immediately undone.
And yeah, the movie deliberately goes out of its way to troll the fans of the original (and even explicitly says it does in one of the fantasy sequences), but unlike other movies that do this (Gremlins 2, Matrix Resurrections), there's nothing interesting going on behind it. It's a vapid movie made by vapid people for no one.
I was not a fan of the first movie (in fact, I think that movie is also bad), but at least it had ideas. Folie a Deux has nothing.