Ultimecia made Edea look that way (the helmet) and her clothes. There's evidence to support this within the game
Of course she did. She was possessing her.

when Edea got rid of Ultimecia, she no longer holds the Helmet; she's back to Matron herself (long black hair and freer clothes).
She looks exactly the same to me when she joined your party and was at the lighthouse with Cid.