They do.

If they weren't evil clowns or playing tricks on us, they would have had ended everything without any problem. How can tiny particles stop that? I know that it wouldn't be a story/game if that was truly the case, but still.

What are the possibility and probability of such very powerful and gigantic spells being stopped by such tiny particles (characters)?
Very extremely low, I'd say.

Both Ultimecia and Kefka have represented a style of 'clown' in one way or another. And plus both of thier spells are equivalent to each other. Therefore, the connection of Ultimecia and Kefka isn't a coincidence.