It is the punishment of bad princes to be thought worse than they are.

[Le châtiment des mauvais princes est d’être crus pires qu’ils ne sont.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 14 “Des Gouvernements [On Governments],” ¶ 15 (1850 ed.) [tr. Collins (1928), ch. 13]
    (Source)

(Source (French)). Alternate translations:

The punishment of bad princes is to be thought worse than they are.
[tr. Calvert (1866), ch. 12]

The punishment of bad princes is to be thought worse than they really are.
[tr. Attwell (1896), ¶ 195]