I would easily say he would have sounded a lot more like Mark Hamil as the Joker then Heath Ledger. No way could I hear Dark Knights Batman doing Kefka's signature laugh, something the animated series Joker could probably have pulled off better.
That asside, Kefka was wild, insane, and very over the top about it. A higher tone voice suites him perfectly. I still feel they could have done better, but it's an acceptable choice.