First consider Victor's words about his first witness to the creature he has created:
"How can I describe my emotions at this catastrophe, or how delineate the wretch whom with such infinite pains and care I had endeavoured to form? His limbs were in proportion, and I had selected his features as beautiful. Beautiful!—Great God! His yellow skin scarcely covered the work of muscles and arteries beneath; his hair was of a lustrous black, and flowing; his teeth of a pearly whiteness; but these luxuriances only formed a more horrid contrast with his watery eyes, that seemed almost of the same colour as the dun white sockets in which they were set, his shrivelled complexion and straight black lips.
The different accidents of life are not so changeable as the feelings of human nature. I had worked hard for nearly two years, for the sole purpose of infusing life into an inanimate body. For this I had deprived myself of rest and health. I had desired it with an ardour that far exceeded moderation; but now that I had finished, the beauty of the dream vanished, and breathless horror and disgust filled my heart. (Chapter 5).
Here we see a contrast between what Victor envisioned--a beautiful dream about overcoming death, designing this newborn, manmade human being who would be immune to death, angelic--and what actually was created--a reanimated jigsaw puzzle of various corpses. We see directly in the quote how swept away Victor became in the beauty of his idea; he was so convinced that what he was doing was right that he was able to reassemble dead bodies rather than sleep! He even intentionally picked beautiful features for his monster, like lustrous hair and a muscular physique, but he failed to acknowledge to monstrosity of the end product. He didn't anticipate how unnatural, confused, and terrifying the "monster" would be. The monster's appearance is hideous, but so is Victor's action of bringing it to life without thought of consequence.
This is paralleled as the book goes on as well, where we see some true beauty in the monster's inner being that contrasts with his grotesque appearance. It is only through Victor's ugly rejection that that beauty is darkened and deformed.