I always thought that Robin Williams wanted to play the Riddler, but they got Jim Carrey instead because he was a rising star.
I think either, really, would have been (was) a terrible idea.
I don't know if I'd go that far, but I do think it was a mistake to approach the Riddler as a Joker clone, a cackling maniac. The Riddler should be a more cunning, intellectual foe, a villain with a certain degree of class to him (his original appearance was modeled on Fred Astaire, after all). After all, jokes are about making you laugh or groan, but riddles are about making you think, perplexing you with puzzles and mysteries. Batman is the World's Greatest Detective, so the Riddler should be a source of intricate conundrums for him to pit his mind against. (Although that's rarely achieved, since not a lot of comics or TV writers are smart or devious enough to come up with really good riddles or puzzles. The first two B:TAS Riddler episodes gave poor Eddie really lame material. At least
Batman Forever had the sense to hire Will Shortz to concoct the riddles, though they were a bit anticlimactic given that.)