The phrases can be reordered. Alternate translation: "A proverb in the mouth of fools is like the legs of a paralytic which hang down" or "A proverb in the mouth of fools is as useless as the legs of a paralytic which hang down" (See: [[rc://en/ta/man/jit/figs-simile]])
Here "mouth" is a metonym for speaking. Alternate translation: "in the speech of fools" or "that fools say" (See: [[rc://en/ta/man/jit/figs-metonymy]])