In the 1970s there was a song “Junk Food Junkie” about a man who espoused the evils of junk food by day and by night snuck across town for a “fix” of Twinkies and ate them when no one was looking. Eating junk food is obviously not sinful, if done in moderation. In this case, saying one thing (it’s wrong to eat junk food) and doing another (eating it on the sneak) is not honest. But is it a mortal sin, i.e., must it be told in confession?
It depends on how serious the gravity of the situation is, which can depend on a multitude of factors. For simplicity’s sake, minor hypocrisy without just cause may be venial; major hypocrisy may be mortal.
You may wonder why I added the clause “for just cause.” Not all that is sometimes considered “hypocrisy” is sinful at all. For example, telling people who ask “How are you?” that you’re “fine” even while you have a headache is not sinful. Part of the reason for that is because “How are you?” is conventional conversation and not a serious inquiry into the state of one’s health. If there is just cause to use a platitude – for example, to protect privacy or to avoid unnecessary argument or to avoid hurt feelings – it is not wrong to do so even if your true feelings conflict with the platitude.
Only if there is not just cause to use social platitudes might there be culpability for hypocrisy. As a general rule of thumb, hypocrisy can be sinful when it unjustly misleads others or when it deliberately causes harm to others.