And he walked in all the sins of his father, which he had done before him: and his heart was not perfect with the LORD his God, as the heart of David his father. His heart was not perfect - He was an idolater, or did not support the worship of the true God. This appears to be the general meaning of the heart not being perfect with God.
Related Commentaries of 1 Kings 15:3
1 Kings 15:3
3 And he walked in all the sins of his father, which he had done before him: and his heart was not perfect with the LORD his God, as the heart of David his father.