I honestly don't understand the "it's just a movie about a dumb loser" sentiment. Unless maybe you just watched the first half of the movie or so. He is a much different person at the end of the movie than the beginning.
He is clearly completely crippled by a horrific upbringing and all the baggage that comes with it, mentall illness, and being on SEVEN different drugs. He hasn't had an ounce of happiness or confidence his entire life...much less any taste of power, control or influence. He's a messed up 40-something year old man who lives with his crazy mom (who we later find out is not his mother) and bathes her. He was abused and neglected in ways none of us can imagine. And yes, he can't spell for sh**....OK, I guess he is dumb in many ways, fair point.
But as the haze is lifted (partly by stopping his meds cold turkey I assume) and he begins feeling joy and pleasure for the first time by acting out and enacting psychopathic revenge on the "cruel" world, he starts becoming smarter and more calculated. He's still a pretty clumsy criminal, with a lot to learn, but you can see him trending towards the Joker we are all more familiar with as the movie goes on. He feeds off notoriety and finally being noticed for the first time in his life. His violence has started an antifa/occupy-wallstreet like movement.
I am expecting him to achieve "diabolical genius" status in the second one, personally. If done correctly, I think it might help many people view the first one in a better light in hindsight.
Regardless, I hope we can all agree that he is much more unique and nuanced than this one...