。
There's no fool like an old fool.
Literal
Old-man [possessive-の] fool [degree-ほど] foolish thing [topic-は] not-exist.
~ほど~ない ('nothing is as X as Y') is a superlative pattern. This is the Japanese rendering of the English proverb 'there's no fool like an old fool.' The repetition of ばか/ばかな drives the point home.