I'm not sure why anyone thought Eric Roberts as a slimy mobster was going to be a stretch (that's 2/3 of his filmography), but I thought he did fine, and wish he could have had a little bit more screentime.
I enjoy Eric Roberts in TDK the same as I enjoy another B movie legend getting a rare return to the spotlight, Rutger Hauer in BB. They're not "great" actors, but they're fun to watch, with the little tongue-in-cheek campy touches they throw in (we all know Hauer's a scenery-chewer, and does anyone notice when Rossi is about to shoot Dent in court, in the background, Roberts like pumps his fist in the air with this "YEEEAAAHHHHH!!!!" expression. lol. Having fun Eric?)
He was actually really funny on that Gotham Tonight episode (kinda funny how much more Anthony Michael Hall had to do in the virals than in the actual movie), and the nightclub/"where is he!" scene is one of my favorites in the movie. That was kind of Roberts' "big" moment in TDK, and he did good.