Mad Hatter works best for small stories, either as a foe of a sidekick, dealing with girls on their age level, or for a small contained kidnapping-type story.
Mad Hatter's tech doesn't really make sense to me, and it's very sci-fi, but not necessarily based on real science. I mean, real subsonic hypnosis could be, theoretically, broadcast, but his hats are often treated like 'magic' and it makes for lesser stories.
I think the Mad Hatter in Robin: Year One should be built on. He's a pedophile, child trafficker. He's way deep into the dark side, into icky territory. He's a shock jockey, a bit like the Joker can be, but instead of balancing and becoming empathetic via humor and antics, the Mad Hatter would probably work better as a remorseless business man, or a introspective philosopher who sees himself as trading on dreams or people as easily as he'd sell a toaster.
As is, his coupe de grace is turning innocents against the Batman, which doesn't really have a lot of pull, unless he's brainwashing people Batman cares about against him. I haven't seen that done. They won't give him the respect he needs to be allowed to step on Batman's toes, and thusly earn the necessary respect to do so.