What is the ruling on calling a person who enjoins good and forbids evil a "lying muttawi'"?

It is not permissible to describe a Muslim as a liar, especially one who enjoins good, as this is among the forbidden acts in Islamic law. It falls under harming believers, calumny, and manifest sin, and it is considered فسق (disobedience/wickedness) as stated in the noble Hadith. If a person describes someone who enjoins good upon them as a liar, in order to reject the advice, then they have fallen into rejecting the truth and disdaining it, which is among the grave sins.