Hoo boy.
A lot of these complaints seem to stem from the fact that The Joker, whoever he is, will somehow be a copycat in this version of the story.
Newsflash, people. The Joker already is a copycat, to an extent. He has never been the most original character. He's a bad comedian, he reuses jokes, he makes obvious jokes, he uses the somewhat generic motif of a clown...it's not like he's the first character in literature to kill people, or to enjoy doing it...or the first villain to laugh maniacally...etc.
While Jerome laughed and smiled and seemed to enjoy what he was doing, and had a sense of showmanship, that does not, in itself, make him all the things The Joker needs to be. It's the seeds of The Joker, yes, but obviously not a fully formed character, or concept.
What makes The Joker who he is, are his actions and the specifics of them, not his most generic traits (laughing, enjoying killing).
Jerome was something of a proto-Joker to be sure. One step toward the legacy of insanity and villainy that The Joker will capitalize on eventually.
The actions of Jerome and the Maniax are going to inspire a whole new generation of psychos and sadists, as we saw at the end of the episode. In other words, GOTHAM is setting the stage for a city that NEEDS someone like Batman in the future. A city where the emergence and existence of theatrical "themed freaks" and ultimately their clown prince, isn't completely out of place.
Does that mean we'll see possible Jokers, to keep the audience guessing? Probably, but they'll be more "crazy killers" than outright wannabe Jokers, I think. I think it's a neat idea, myself.