handsome is as handsome does
handsome is as handsome does {informal}
A person must act well and generously so that he will be truly worth respecting. — A proverb.
Everyone thinks that Bon is a very handsome boy, but he is very mean too. Handsome is as handsome does.
Compare: FINE FEATHERS DO NOT MAKE PINE BIRDS.
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Source: A Dictionary of American Idioms