Can a Catholic criticize the Pope? Or is it a mortal sin to do so?
No. Not necessarily.
Catholics are obliged to have filial love for and obedience to our Holy Father. Neither that love nor that obedience are required to be blind or stupid.
Criticism of the Pope can become a mortal sin if one’s criticism is filled with a hatred and vitriol that shows a lack of respect or filial love for Our Sovereign Pontiff. One must also consider to whom you show that lack of respect. If by your words and actions you harm his reputation with others unjustly, you do him and them a grave wrong. You also may be committing the sin of sacrilege.
The Pope is Christ’s Vicar, and deserves all the respect of that office.
The Pope is, however, not Christ. Nor does his charism of infallibility render him perfect in all his words and actions.
He may do things that are objectionable. When he does, he can be criticized – respectfully.
But be careful in aiming criticism at the Pope. Be careful to whom you open your mind or reveal your attitude. Examine your conscience with brutal honesty, remembering that His Holiness has a perspective on the Church that we do not.
Catholics loves their Popes. That doesn’t mean that we always like them or everything they do.
We should, however, avoid giving scandal. Maintain respect for the Holy Father when speaking about him to others, heed his words on faith and morals, and give him obedience when it is called for.
You can bet that for this one the moderation queue is ON.