Okay, here's my attempt to explain why Boss Maroni disfigured Harvey Dent with acid in the comics.
Boss Maroni was finished. His crimes were exposed, he was going down. So he agreed to make a deal with Harvey Dent, exposing Carmine Falcone's illegal activities in court.
HOWEVER, Sofia Gigante - Falcone's daughter - was able to make contact with Maroni in his holding cell. She convinced him that this was not the way to go down, for his family to be remembered. If the families were to ever be united again, Maroni had to make a bold, public demonstration of his loyalty to Falcone. And what better way to do it than by making an example out of one of his top enemies - Harvey Dent - in open, public court? The statement would be simple: this is what happens when you try and bring Falcone to justice.
Hence the acid. Easier slipped into the courtroom than a gun (it was disguised as cough medicine), and plus, throwing it in someone's face would surely make for a horrible, agonising death. Like others have said, the acid was meant to kill Dent. But the attempt failed, and instead he was turned into Two-Face.
Of course, for the movie, this would have to be simplified. Maroni's motive for doing it could be changed, removing the Falcone elements and replacing them with simple revenge.
But at the same time, I wouldn't have a problem with The Joker being the one who scars Dent either.