You could have named the guy anybody else, and the movie could have held it's own. The reason why this movie was so successful is because it has a sympathetic main character, and it deals with the social environment we currently live in with the divided classes of the rich, poor, and entitlements. This movie just elevated those traits of the movie to make it more "grittier". Otherwise, calling this movie "Joker" seemed more like a marketing ploy to give people familiarity to a character they loosely know from DC comics and as Batman's main arch nemesis.
As for the main character himself, I felt that in no way he could have evolved into the criminal mastermind we know from the comics or from the Dark Knight. The things he did in the movie we either out of self defense, or he was lashing out at bullies... Not because he was scheming or plotting some master stroke plan. Everything was a reaction to that moment. Was he crazy, yes. But I wouldn't consider his type of mental illness to be aggressive in any way, shape or form. Him and the Dark Knight Joker are two different monsters, whereas I would consider this Arthur a very passive and submissive loner that reacts to his current environment, whereby the Dark Knight Joker is more passive aggressive with an intent of malice in everything he does.
In all, I thought the movie was okay. It immediately reminded me of that 1993 movie "Falling Down", with Kurt Douglas, where he was taken advantage of throughout the movie, and found courage through self-empowerment.