It is what all comic movies are, a film adaptation of comic book lore, using the comics as a foundation upon which to tell a story via the cinematic experience.
As different of a movie as it is from your typical Avengers and Batman adventures, it would be a disservice to Joker to entirely separate it from any other comic book adaptation. There are so many references to things you are familiar with from comics, it would be disingenuous to say it's not a comic adaptation.
I watched this movie and halfway through it I thought to myself, "I see why Joaquin Phoenix turned down Doctor Strange." He didn't mind doing a comic book movie but not a virtually empty one that would be discussed today and forgotten tomorrow. This film highlighted some very important facets of society and life that it is, almost instantly, timeless.
This is a must see and on first watch, I give it a 9.5/10.