It is the habit of mediocre minds to condemn all that is beyond their grasp.

[Les esprits médiocres condamnent d’ordinaire tout ce qui passé leur portée.]

François VI, duc de La Rochefoucauld (1613–1680) French epigrammatist, memoirist, noble

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶375 (1665–1678) [tr. FitzGibbon (1957), ¶375]
    (Source)

This adage first appeared in the 5th (1678) edition. See also ¶265.

(Source (French)). Other translations:

Men of Indifferent Parts are apt to Condemn every thing above their own Capacity.
[tr. Stanhope (1694), ¶95; Stanhope (1706), ¶508]

Minds of moderate calibre ordinarily condemn every thing which is beyond their range.
[ed. Gowans (1851), ¶379]

Ordinary men commonly condemn what is beyond them.
[tr. Bund/Friswell (1871), ¶375]

A mind of mediocre attainments condemns everything beyond its scope.
[tr. Heard (1917), ¶376]

Minds of average capacity generally condemn what is beyond their grasp,
[tr. Stevens (1939), ¶375]

Second-rate minds usually condemn everything beyond their grasp.
[tr. Kronenberger (1959), ¶375]

Commonplace minds usually condemn whatever is beyond their powers.
[tr. Tancock (1959), ¶375]

Mediocre minds ordinarily condemn anything that passes beyond their reach.
[tr. Whichello (2016), ¶375]

Mediocrities usually condemn what is beyond their capability.
[tr. Chandler, ¶375]