“Some men are born mediocre, some men achieve mediocrity, and some men have mediocrity thrust upon them.”

Quote magazine (15 June 1958)
1950s
Context: When human beings have been fascinated by the contemplation of their own hearts, the more intricate biological pattern of the female has become a model for the artist, the mystic, and the saint. When mankind turns instead to what can be done, altered, built, invented, in the outer world, all natural properties of men, animals, or metals become handicaps to be altered rather than clues to be followed. Women want mediocre men, and men are working hard to be as mediocre as possible.

Source: To the Hilt (1996)

Malvolio, Act II, scene v.
Variant: Some are born great, others achieve greatness.
Source: Twelfth Night (1601)

Source: Pène du Bois (1897), p. 99.

La modération des grands hommes ne borne que leurs vices. La modération des faibles est médiocrité.
Source: Reflections and Maxims (1746), p. 168.

Lines 372–373 http://books.google.com/books?id=hlgNAAAAYAAJ&q=%22mediocribus+esse+poetis+Non+homines+non+di+non+concessere+columnae%22&pg=PA769#v=onepage
Ars Poetica, or The Epistle to the Pisones (c. 18 BC)

Source: Bitter of Tongue

Source: God Emperor of Dune