Not all flat feet are ugly. Let's give an example of flat feet that are beautiful.
Flat as can be, but beautiful! There are many more examples of beautiful flat feet.
Just tickle her will you? If she'd take care of her feet, they should be beautiful anyway. Also, the feet are not the only part of the female body that can look gorgious.
I personally know a beautiful girl who has lizard-like skin on her arms, hands and back and due to a gas accident in 2004 (she had third and second degree burns). How terrible the wounds may be, she still remains beautiful as ever and she isn't my girlfriend or whatsoever, not even a close relative. She just was beautiful in my eyes from day one.
You just cannot say that one particular detail makes someone ugly, because you'd probably meet people who have that same "feature", who are nice in a way you haven't thought of.
Just give the girl a chance and you may be positively surprised and even changes in taste may develop (often takes a week for me, when I meet a girl I learned to like, but didn't catch my attention before). It often happened to me, that is.