Of course, the appearance difference between Rinoa and Ultimecia is striking. But answer this question yourself: what did Adel look like before becoming a sorceress? It's very likely that before her sorceress state, she looked much more human; more like Rinoa and other human beings.
I don't think it was Adel's decision to change her own appearance. Why would one want to make itself look more ugly? Because her radio words implied that she wanted to get out of there.
I think her appearance changes gradually when she's being imprisoned for so long. Out of control.
Just like Ultimecia. She and Adel got out of control. Both of them were insane. Adel was imprisoned. So it'd be logical to say that Ultimecia was also imprisoned as well. Ultimecia wanted to erase everything so that she can be free. Adel wanted to get out of there and go free.
You know what's interesting. Edea's appearance changes when Ultimecia possesses her. It definitely wasn't Edea's decision to change her own look; it was Ultimecia's decision. Ultimecia also got these lions and stuff on the Garabian garden; the same stuff she uses for her castle.
But the interesting part is that Rinoa's appearance didn't change when she was possessed by Ultimecia. There's no good reason to leave her appearance intact unless she's Ultimecia herself. Or was it because... when Rinoa is being possessed, Ultimecia couldn't concentrate properly because she felt the strong love between Rinoa and Squall? Love? Would that explain the crazy walking that Rinoa had when she was possessed - during the parade party, Rinoa and Adel were both possessed by Ultimecia at the same time. Edea could speak Ultimecia's language and remain... physically stable, however Rinoa wasn't physically stable and she couldn't even talk. Or it's simply because there's something special about Rinoa that many other sorceresses don't have.