It wouldn't be too hard to portray Mxy as a malevolent trickster with otherworldly abilities while not making him too off-the-wall - it's all in the characterization. You wouldn't even have to make him darker or edgier, or maybe not even necessarily less humorous. Just don't make him or his presence quite as zany in terms of the way he works - if you want to keep a more "serious". Play him as a superpowerful being testing Supes for his own amusement, who then turns nasty when Superman starts beating him. Have his joking behavior be quite clearly a form of mocking and condescension.
It's always bothered me how people equate unorthodox to "silly," and then go from "silly" to "unworkable."
It would, however, require writing a movie where the villain is pretty much totally separate from the rest of continuity - Mxy would be really
hard to introduce while tying to the universe as it exists, even if you weren't going by a film continuity or not (This is actually the biggest thing going against him for being a main villain): Mxy by nature comes out of nowhere and goes back just as suddenly, often without far reaching consequences after he's gone. He has no inherent ties to Superman's mythos, and is from a realm totally separate from Supes or his aspirations. It kind of works against him terms of "film Big Bad
" status, because it comes with a lot of Fridge Logic
Also, being tricked back to his realm by saying his own name backwards isn't really a bad trigger for defeat, but for a feature length film it feels a bit to... simple. Like it would make the end of the climax far too abrupt. It definitely works better for something shorter, where Mxy can reappear in short order, but a film would require something a bit different. Then again, he's had several other triggers in his long existence (IIRC saying his own name backwards was a rule he decided to adopt after being prompted, the main reason he kept with it so long was because he didn't like lying (or something), and he could change it if he was really inclined to do so).
edited 9th Dec '12 3:05:19 PM by KnownUnknown
"My final prayer: O my body, always make me a man who questions!" — Frantz Fanon