A plain appearance is to ordinary men their proper garb: it suits them and fits them, but it adorns those persons whose lives have been distinguished by grand deeds; I compare them to a beauty who is most charming in négligé.

[Un extérieur simple est l’habit des hommes vulgaires, il est taillé pour eux et sur leur mesure; mais c’est une parure pour ceux qui ont rempli leur vie de grandes actions: je les compare à une beauté négligée, mais plus piquante.]

Jean de La Bruyere
Jean de La Bruyère (1645-1696) French essayist, moralist
The Characters [Les Caractères], ch. 2 “Of Personal Merit [Du Mérite Personnel],” § 17 (2.17) (1688) [tr. Van Laun (1885)]
    (Source)

(Source (French)). Alternate translations:

That Simplicity of outward Appearance, which in vulgar Men seems to be their proper Clothes, shap'd and fitted to their Size, is the ornamental Habit of those Persons whose Lives have been full of great Actions. I compare 'em to a Beauty, that is more charming for being negligent.
[Curll ed. (1713)]

A plain Exterior is to ordinary Men their proper Garb, shaped and fitted to their Size, but is an ornamental Habit in those Persons whose Lives have been distinguished by signal Actions. I compare them to a Beauty, most charming when à la negligé.
[Browne ed. (1752)]

Outward simplicity befits ordinary men, like a garment made to measure for them; but it serves as an adornment to those who have filled their lives with great deeds: they might be compared to some beauty carelessly dressed and thereby all the more attractive.
[tr. Stewart (1970)]