What would be some good points to discuss in an essay on the theme "fair is foul and foul is fair" as it concerns Macbeth and Frankenstein?

Your general themes are appearance versus reality and good versus evil. Frankenstein and Macbeth approach these themes quite differently, so you might want to structure your essay to compare and contrast them.
Macbeth shows the transformation of an admirable character into an evil one through a combination of the blandishments of the three weird sisters and his own ambition. Over the course of the play, Macbeth degenerates from a "fair" hero whose ambition, bravery, and energy have caused him to be a powerful and admired supporter of King Duncan into a "foul" tyrant and murderer. Although Macbeth himself changes over the course of the play, readers are not asked to think about the nature of our own moral judgments. We are expected to share a moral stance that regards murder and treachery as evil. In the play, fair becomes foul and Macbeth's own perceptions distort.
In Frankenstein, we are confronted with the dilemma of whether, our perception of fair, as seen in the educated European Victor, and foul, as exemplified by the monster, should be reversed. We need to consider whether the fair and civilized scientist is in fact more foul than his monster, who appears hideous on the surface, but is really as much a mistreated innocent as a source of evil. The novel also asks us to question whether we should blame mistreated outcasts for lashing out at society or whether the society that has mistreated them is really the "foul" entity that is morally culpable.