Adel and other sorceresses likely have the power to change their form after becoming more acquainted with their power. Sorceresses like Edea and Rinoa that choose to pose no threat to ordinary people have little reason to do this other than for cosmetic reasons.

The Final Fantasy Wiki has a great article on Adel.

I thought that one of the most interesting parts was the following:

Adel only speaks three lines throughout the entire game: "What is going on?" "Where?" and "You thought I would fall for that trick?" - however, on Squall's first visit to the Timber TV Station, several repeating phrases are shown on the huge screen, including "IAMALIVE" (I am alive), "IWILLNEVERLETYOUFORGETABOUTME" (I will never let you forget about me), and "BRINGMEBACKTHERE" (Bring me back there). These are undoubtedly the words of Adel, whose tomb in space is the cause of the worldwide radio interference down on the planet.