purely from random osmosis, but if bill cipher was born with his eye on his "side" (facing upward in what Flatlanders call their inside), then he'd be blind as a Flatlander because we wouldn't be able to see within Flatland itself at all.
And also no one would even be aware that he has an eye to begin with, because it's literally outside their realm of perception. They'd think he was born without any eye and that he was literally just imagining anything he thought he could see.
[ID: A four way grid with the first row labeled "Bill Cipher apparently", first seen from above, as an equilateral triangle with a round eye in the center of his face, looking up and to the side. He is then seen from the side as a line with a half circle on top to show his eye, labeled, "Cannot see surroundings at all, cannot look in any direction except Up". from his eye radiate three lines to show his area of vision, with a zig-zag line in front of him to show where he cannot see. The second row is labeled, "Regular Flatlander", and shows another equilateral triangle, but this one has their eye on their perimeter at their top point. When they are shown from the side, they are labeled, "Can see surroundings, but cannot look Up", with three straight lines in front of their eye, which points to the size, and a zig-zag above them. End ID.]
So. If you're planning to write a crossover with Flatland where you want it to actually follow Flatland's worldbuilding, Bill should be blind while he's in Flatland. He'd have to navigate purely though touch and sound and would never see anything except whatever is above him.
Edit 3 hours later: also, he would have just starved to death as a baby unless there were some serious and immediate medical intervention, because Flatlander's eyes and their mouths are the same thing. So he'd have no way of eating anything.
So, he would also need the Flatland equivalent of a feeding tube for his whole life, unless he had some guardian angel in Spaceland who was feeding him from above.