Shelley isn’t big on the subtle showing. She pretty much tells you what characters are thinking or feeling. With all the complicated symbols and duality and "what does it mean to be human?" questions, we kind of appreciate her tossing us a bone on this one.
Well, identifying "looks" as a tool of characterization in Frankenstein is kind of like shooting immobile fish in a tiny barrel. While we, the reader, understand that the monster is ugly, it’s actually the characters themselves that use looks as a tool of prejudice and unwarranted characterization. Since the monster is ugly, he must be evil. The fun part comes in when the monster actually does become sort of evil, or at least commits evil crimes. He conforms to everyone’s expectations. He becomes the victim of his own characterization.