I believe that Boa Hancock's unmatched beauty is the result of her Devil Fruit powers.
Allow Me To Explain:
The Love Love Fruit gives the power to turn anyone who feels attraction towards the eater to stone.
Its powers wouldn't be very effective if the eater's looks were "average".
Also, certain Paramecia Devil Fruits in the past have had body altering properties: Luffy's rubbery body, Alvida's instant weight loss, Daz Bones' hard as steel skin, and Sugar's eternal Youth. So it is just as likely that Hancock's DF could be capable of something similar.
And while it IS true that Hancock doesn't appear to look too different from when she was younger, when your beauty can attract anyone regardless of age or gender, I'm pretty sure that it borders on supernatural.
What do you think?