The tragedy in The Crucible lies to an overwhelming extent in the moral weaknesses of its characters. Is this valid?

This is very valid. The theme of moral weakness manifests itself in a number of ways. John Proctor succumbs to the sexual temptations of a seventeen year old girl. Abigail is able to exploit moral weakness in just about everyone she meets. Abigail is the catalyst for the collective moral meltdown of the town. From there other characters seem to revel in their breakdown of morality. Rev. Parris's true colours really show as he seeks to protect his reputation, job and money over the lives of innocent people in his parish. Putnam goes on his major land grab, Mary Warren can't find the courage to do what is right....Indeed the weakness in human morality, not religious morality, is at the heart of this play.