"Remember how in that communion only, beholding beauty with the eye of the mind, he will be enabled to bring forth, not images of beauty, but realities (for he has hold not of an image but of a reality), and bringing forth and nourishing true virtue to become the friend of God and be immortal, if mortal man may."
-- Plato, from his Symposium

The phrase "Beauty is only skin deep" probably didn't come from Plato, although he is often credited as the first to express the idea. Something approaching the modern form was first used by Thomas Overbury (1581-1613) in his poem "A Wife".

'All the carnall beauty of my wife, Is but skin deep'.

The first written instance of the current form in its exact wording was in 1878, when Margaret Wolfe Hungerford (AKA The Dutchess) used it in her book Molly Bawn.

A few quotes:

People often say that 'beauty is in the eye of the beholder,' and I say that the most liberating thing about beauty is realizing that you are the beholder. This empowers us to find beauty in places where others have not dared to look, including inside ourselves. - Salma Hayek

The saying that beauty is but skin deep is but a skin deep saying. --John Ruskin

Beauty is only skin deep, and the world is full of thin skinned people. -- Richard Armour

Beauty is only skin deep, but it's a valuable asset if you're poor or haven't any sense. -Kin Hubbard

Competence, like truth, beauty and contact lenses, is in the eye of the beholder. -Laurence J. Peter, creator of The Peter Principle

I'm tired of all this nonsense about beauty being only skin deep. That's deep enough. What do you want -- an adorable pancreas? -- Jean Kerr

Beauty is in the eye of the beholder and it may be necessary from time to time to give a stupid or misinformed beholder a black eye - Miss Piggy

Bitter_engineer's quote, "Beauty is only skin deep--but ugly goes clear to the bone" is often called Parker's Law.