I think that in a way Frankenstein is projecting how he feels about himself when he looks and thinks about his creation. He feels like he himself is the monster for having created such a thing. Part of his growing disgust in it stems from the fact that he knows he has killed William and knows that he’s the one to blame for it. Throughout the monster’s plea for Frankenstein’s help in making him a partner of the opposite sex, he switches up a lot, going from understanding back to disgusted and unwilling to help. Even before the monster killed William, if he had created a more “Normal” looking subject he most likely would not have fled and abandoned it. Frankenstein would’ve been trying to show the whole world what he created and try to monetize or gain fame from it.
Throughout the creature’s plea Frankenstein acknowledges that it can articulate its feelings and thoughts in a “normal” manner. However, it’s always its looks that throw him off at the end. He admits that its “Tale and the feelings he now expressed proved him to be a creature of fine sensations,” (Chapter 17) But when Frankenstein “Looked upon him, when I saw the filthy mass that moved and talked, my heart sickened, and my feelings were altered to those of horror and hatred”. (Chapter 17) This shows how he is just being ableist and judging his creation based on its looks. The value he puts on the monster is based on the outside, not on what it is capable of doing, saying or feeling which is far more impressive than just focusing on its appearance.