The Joker is, by definition, a superhero movie because it takes place within the universe of a DC property. It (re)tells the origin story of one of the most famous comic book villains of all time.

It may be stretching the boundaries of the genre, but it's still in the genre.

We might disagree forever, however this is my argument. What connects the Joker movie to the Batman universe are essentially three names: the Joker's himself, Bruce Wayne, and Gotham City. And names are labels: you can stick the label of a bottle of wine on a bottle of coke, but it remains a bottle of coke: it's sweet, fizzy, and won't make you drunk.

If you take a Harry Potter novel and rename him as Batman, his enemy Voldemort as Joker and Hogwarts as Gotham School, it doesn't make it into a DC superhero story. If you take a fantasy novel and replace kingdoms with planets and castles and horses with starships, it doesn't make it into a science fiction story- despite Star Wars being labeled as one.