The Manga shows her to be quite attractive in fact.

Do you need me to count for you the number of anime villianesses who are
attractive and deadly at the same time? Mara's one you know.
You are quite shallow if you think attractiveness is the basis for a match
up. Give me a better idea.

"Now Mara, that isn't a nice way to greet someone higher than you.

Mara IS a 1st Class Unlimited Demon as well.