On chapter V, Victor says about the monster he has just created:
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.
From my point of view, it could as well be that he is surprised at how little skin covers his innards, but I am more partial to the eyes, as he describes them quite creeply when he says "seemed almost the same color as their sockets".
You can also argue about the eyes because that was the first thing that Victor saw face to face, as the first eye opened.
It was already one in the morning; the rain pattered dismally against the panes, and my candle was nearly burnt out, when, by the glimmer of the half-extinguished light, I saw the dull yellow eye of the creature open; it breathed hard, and a convulsive motion agitated its limbs.
So, deductively, I think the shock of seeing his eye open and then seeing how they look literally like the eyes of a zombie would freak him (and me) out a lot.
Victor Frankenstein in forming his creature had not been careful to make sure that every gap of skin was closed up. The eyes are dull yellow and do not look alive but are alive. His skin is yellow and hangs funny onto the body's muscles. His hair is a beautiful lustrous black and flows nicely, but his eyes are watery and grotesque. His complexion is shriveled and he has black lips. The creature is anything but beautiful.
Victor had expected his creature to rise up and be a man, but the creature is merely a combination of parts. Although it is alive it looks dead and walks. It is like a mummy. He mae it eight foot tall and grotesque. To Victor it is a thing from "Dante's Inferno."