Here the word "lips" is a metonym for what the king says. Alternate translation: "What a king says are insightful decisions" (See: [[rc://en/ta/man/jit/figs-metonymy]])
The word "mouth" represents the king himself. Alternate translation: "he should not speak deceitfully when he judges" (See: [[rc://en/ta/man/jit/figs-synecdoche]])