I'm thinking about it...
"Не по хорошу мил, а по милу хорош" and "beauty is in the eyes of the beholder" aren't equal by implication.
I mean it is better to use "У каждого свое представление о красоте" (we often say that) for "beauty is in the eyes of the beholder".
And try to avoid "Не по хорошу мил, а по милу хорош" and "Красота в глазах очевидца". As I've said they are too vague to me