"Because, he's an evil clown" just doesn't hold up to the background stories of Sephiroth, The Sorceress, Kuja, or Sin.
A villain doesn't need a ten thousand word backstory to be an decent villain. In the case of Kefka, him prancing around doing a tone of evil s@#$ with childish glee is enough to make him a very effective antagonist.
I was referring to the poster who said that the first generation of Final Fantasy had much deeper and better stories than the second. I'm not bashing Kefka, to this day his theme music still occasionally gives me goosebumps.