"Dr. Heidegger's Experiment" is rooted in a rather pessimistic view of human nature. The story argues that people are, for the most part, fools. They don't learn from their mistakes, they're generally petty, and we can't expect anyone to change for the better. In this story, foolishness is particularly associated with youth, or at least a youthful state of mind. Hawthorne does provide a counter-example to his foolish characters in the form of Dr. Heidegger, but even this character has his sinister side.
In illustrating the foolishness of his characters, Hawthorne condemns his readers as fools as well.
The four guests are two-dimensional caricatures, not fully developed, three-dimensional characters.