Joker was really close to being a great movie, but was ultimately too confused about what it wanted its message to be. Unless the message was just "I'm fed up, let's get violent already".
That's the problem, it didn't have a real message. It was aping Taxi Driver and King of Comedy, and it copied the aesthetics of those films very well, but it was all superficial. It's a perfectly fine movie, but The Batman had deeper social commentary than Joker.
Because it tried and failed to say something profound about mental illness while imitating one of the most celebrated directors in American cinema. Not sure why you're pretending this was just another superhero blockbuster when it was clearly trying to be an Arthouse film