In an interview with BBC (via Indiewire), Scorsese talked about why he eventually passed on the project and if Joker falls under the ‘superhero’ genre.
Before people start yelling at Scorsese again, he isn’t saying that superhero movies suck. He’s basically just saying it isn’t for him and it’s disappointing that the rise of the superhero genre has also caused films like The Irishman to not get a wide theatrical release. In fact, there wasn’t even a studio that wanted to produce the film until Netflix eventually agreed. Joker is now the highest-grossing R-rated film of all time. The film is still playing in Malaysian cinemas.