But the foolishness of fools is folly - The Targum reads, The honor of fools is folly. The fool, from his foolishness, produces acts of folly. This appears to be the meaning.
“The crown,” i. e., the glory of the wise man constitutes his wealth. He alone is truly rich even as he alone (compare Proverbs 14:18 note) is truly king.
The seeming tautology of the second clause is really its point. Turn “the foolishness of fools” as you will, it comes back to “foolishness” at last.