Only mediocrity can be trusted to be always at its best.

-Beerbohm, Sir (Henry) Max(imilian)
Quoted in S N Behrman Conversations with Max (1960), but also attributed elsewhere to  Jean Giraudoux and W Somerset Maugham.

The tendency of democracies is, in all things, to mediocrity.

-Cooper,James Fenimore
  The American Democrat,'On the Disadvantages of Democracy'.

Mediocrity knows nothing higher than itself, but talent instantly recognises genius.

-Doyle, SirArthur Conan
  The Valley of Fear, ch.1.

A third-rate political wheel-horse, with the face of a moving-picture actor, the intelligence of a respectable agricultural implement dealer, and the imagination of a lodge joiner†a benign blanköa decent, harmless, laborious, hollow-headed mediocrity.

-Mencken, H(enry) L(ouis)
Of President  Warren G Harding. Quoted in Fred Hobson Mencken:  A Life (1994).

War is a condition of progress; the whip-cut that prevents a country from going to sleep and forces satisfied mediocrity to shake off its apathy.

-Renan, (Joseph) Ernest
La Re¤  forme intellectuelle et morale.

It was the supreme expression of the mediocrity of the apparatus that Stalin himself rose to his position.

-Trotsky, Leon originally Lev Davidovich Bronstein
  My Life, p.501.