So? Those things are just comical. Not everything about a villain has to make him a badass.
The comical side to his character is one of the things that makes him so endearing to players. It wasn't just that he destroyed the world, it was that destroying the world was just a big joke to him.

Sephiroth had a much more detailed backstory
Though true, the way Kefka is developed he doesn't need a back story. We are not meant to identify or emphasize with him in the least. I don't see how fleshing out his past would have added to the game in the least.

He also blew up Saturn and Jupiter, as well as the sun during the final battle.
You can't think that has any relevance other than being a cool battle effect.