I think Kefka's claim to fame was probably that he was basically the first worthwhile villain. Most antagonists at the time barely had a name, let along a personality or (god forbid) believable motivation. Sure being a nihilist still made him basically an unrelatable evil. But he was far from typical

He may not have withstood the test of time, but at that time, he was leagues ahead of everything else on the market. At least from my perspective

It's kinda like how everyone fell in love with Heath Ledger's Joker. It was something we hadn't seen done before, and it was so well done. It's not so amazing anymore. But at the time, it was arguably pretty phenomenal