Having read Azzarello's 100 Bullets up to 'The Hard Way' trade, Volume 8, I think, I found that this was my only real problem in reading Joker - That there wasn't much that identified this Joker as anything like the DCU version, moreso than TDK's version, and that bothered me, especially as the solicits promised to get inside his mind.
As it is, though, I felt that the Joker could've easily been replaced with a crazier version of Lono, or any one of the numerous 'crazy gangster' types we've seen in crime fiction and movies over the years. I know that Azzarello was obviously aiming for a touch more realism than what we usually get, but I found that even Morrison's 'super sanity' version of the Joker spoke to me more than this version. There was nothing that really made this a 'Joker' book, for me. It was just a crime novel with a crazy gang boss and some Batman characters slotted in to various roles. Even the Dark Knight's appearance, come story's end, made little sense to me.