There are well-dressed foolish ideas just as there are well-dressed fools.
 
[Il y a des sottises bien habillées, comme il y a des sots très bien vêtus.]

Nicolas Chamfort (1741-1794) French writer, epigrammist (b. Nicolas-Sébastien Roch)

Products of Perfected Civilization [Produits de la Civilisation Perfectionée], Part 1 “Maxims and Thoughts [Maximes et Pensées],” ch. 1, ¶ 40 (1795) [tr. Hutchinson (1902), “The Cynic’s Breviary”]
    (Source)

(Source (French)). Alternate translations:

There is such a thing as well-clothed foolishness, just as there are certain very well-dressed fools.
[tr. Mathers (1926)]

There are presentably dressed follies just as there are well dressed fools.
[tr. Merwin (1969)]

There are some well-turned inanities, just as there are very well turned-out fools.
[tr. Pearson (1973)]

There is nonsense that is well said, just as there are fools who are very well dressed.
[tr. Siniscalchi (1994)]

Foolishness can seem very smart and there are some very smartly dressed fools.
[tr. Parmée (2003), ¶34]