Is one who refrains from censuring the perpetrator of a misdeed in the mosque after the prayer, for fear of losing the remembrance of Allah (dhikr) and distracting one's mind from it, considered sinful?

If enjoining good and forbidding evil conflicts with completing post-prayer adhkar (remembrances), then prioritize enjoining good and forbidding evil, because it is the obligation of the moment, especially if no one else is aware of it. You can make up the adhkar after you finish enjoining good and forbidding evil, because they do not expire. Al-Buhuti mentioned in Kashshaf al-Qina' that dhikr after prayer is sunnah, and if it is recited after standing or while leaving, it still accords with the sunnah. If one is preoccupied from it and then remembers it and recites it shortly thereafter due to an excuse, then it is apparent that its specific reward is attained. Ibn Hajar said: Dhikr is not lost by a long interval; what is lost is only its perfection.