After watching a few interviews with Todd Phillips, my guess would be that the Joker is a depiction of social justice warriors. 
"Go try to be funny nowadays with this woke culture. There were articles written about why comedies don’t work anymore — I’ll tell you why, because all the fucking funny guys are like, 'Fuck this shit, because I don’t want to offend you.'"
It’s hard to argue with 30 million people on Twitter, you just can’t do it, right? So, you just go, 'I’m out.' I’m out, and you know what? With all my comedies — I think that what comedies, in general, all have in common — is they’re irreverent. So, I go, 'How do I do something irreverent, but fuck comedy? Oh, I know, let’s take the comic book movie universe and turn it on its head with this.' And so that’s really where that came from."—Todd Phillips
The Killing Joke
"The book explores Moore's assertion that, psychologically, "Batman and the Joker are mirror images of each other" by delving into the relationship between the two. The story itself shows how the Joker and Batman came to terms with their respective life-altering tragedies, which both eventually lead to their present lives and confrontation. Critic Geoff Klock further explained that "both Batman and the Joker are creations of a random and tragic 'one bad day'. Batman spends his life forging meaning from the random tragedy, whereas the Joker reflects the absurdity of life, and all its random injustice".
The torments that the Joker puts Commissioner Gordon through are meant to serve as "proof that there is something buried deep within each lunatic, a nugget of insanity, that is simply waiting for the right moment to spring forth". Unlike the Joker, however, Gordon emerges from his ordeal with his sanity and moral code intact. The story is also famous for changing how the Modern Age of Comics perceived Batman comics by bringing it back to its darker roots. The comic book, however, delves deeper in order to present Batman's own psychology—that he is, in his own way, just as insane as the Joker, and that he and the Joker perceive the world according to differing points of view, with the Joker's interpreted through a joke. 
The Joker serves as an unreliable narrator. He admits to his own uncertainty, as he has disparate memories of the single event ("Sometimes I remember it one way, sometimes another ... If I'm going to have a past, I prefer it to be multiple choice!"), accentuating the comic's depiction of "a world unraveling toward relentless urban violence and moral nihilism ..." 
"It’s up to you how you want to interpret it and experience it. It’s less you being kind of presented with the facts than you being presented with these possibilities."
—Todd Phillips 
Ce la vie.