Let's see what Square has to say about what Ultimecia was doing shall we.
A sorceress trying to change the world by compressing time and taking power from all sorceresses.
Hm. A world. And she even had to use a Plot Device, Elione, to do it.
As opposed to Nihilus who KILLS a planet "with a word."
Oh and her TK will be as to nothing to a guy who pulled a FLEET of ships from a gravity well with his power.
Sorry guys but Ultimecia never came close to absorbing the universe and according to Square, she was only effecting the planet.
Oh and let's not forget how Nihilus moves through space on a ship he holds together with his will, while also keeping the crew alive. He fougth his oppoents like this, while also double-weakened.
As opposed to Ultimecia who lost to 3 kids at the height of her power.
As you can see, Nihilus can kill a planet instantly.
Name me one thing Ultimecia did on that level?