Is smiling and thinking that he who advises one to do something that he himself does not do has "forgotten himself" considered mockery that expels one from the faith?

What was mentioned is not considered mockery that expels one from the fold of Islam. Your criticism of those who commit sins that they themselves forbid is not sinful if it is a thought to oneself, for the Messenger of Allah, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for what they speak to themselves, as long as they do not act upon it or speak of it."

It is important to warn against the danger of the one who advises violating what he enjoins or forbids. Allah Almighty said: "Do you order people to righteousness and forget yourselves?" and Allah Almighty said: "Grievously odious is it in the sight of Allah that you say what you do not do."

Nevertheless, this does not justify abandoning enjoining good and forbidding evil. Indeed, Al-Hasan said: "Satan wished he had achieved this: that no one would enjoin good or forbid evil." And Sa'id bin Jubayr said: "If a person were not to enjoin good or forbid evil until he was free of any fault, then no one would enjoin good or forbid evil."