Paul Dano was laughably bad
The character ticked practically every movie serial killer cliche in the book. Innocent, dweeby-looking appearance? Check. Soft spoken, childlike voice? Check. Uncontrollable fits of joy and laughter upon causing chaos? Check. A motive stemming from revenge at an unjust society where the people he kills somehow "deserves" it? Check.
And on top of that, Paul Dano was just laughably terrible in the role. How anyone can say he's the perfect successor to Heath Ledger's Joker just eludes me. He was so over-the-top and cartoonish to the point that I almost started laughing in certain scenes.
The movie sucked as a whole too, but Dano was the icing on a cake made of shit and piss.