IDK why but I headcanon that Demi-gods look better than the Normal Human/Mortal
Or that When people (Mortal or Demi-God) first Meet Percy They do double takes and are Shook by his appearance and how attractive he is which is why people think he is a god or looks like one
I think demigods, in general, just have more unique features to them. (Unique hair or eyes since their morphology reflects a part of their godly DNA as well)
After all it's not said enough how much half godly DNA affects them really.
But we know it gives them enhanced physical abilities, so it's only natural that their appearance is better than most mortals.
Demigods certainly have a more alluring presence and unique facial features (eyes, hair .etc) due to their half godly nature.
Though really, I think it's more of a case of who the godly parent is more so than demigod children of all gods.
I mean, children of Ares have rather rugged and fierce features that aren't conventionally attractive .
Children of Aphrodite have strangely ethereal sense to their beauty, which might not always come from their appearance, but due to their attitude or voice or other features.
Children of Athena have only their unique grey eyes and so on.
Percy, however, is something else. He resembles Poseidon extremely closely as has been stated by literally everyone at this point and that no other demigod resembles their godly parent as much as Percy resembles Poseidon. It's another one of Amazing Percy things.
Another theory might be the stronger the demigod, the closer their appearance to that of a God. And we know Percy's the strongest, so it goes without saying that all big three children, especially Percy, look more exceptional or godly than others .
But I fully believe in the fact that demigods have such a unique presence that when mortals meet them they are struck by something a bit otherworldly that they can't exactly put a finger on (not necessarily their appearance) which makes humans a bit uneasy in presence of demigods.
The reason this theory works is because over the span of two series we see demigods go among mortals many times but their's not so much as a strong reaction to their appearance as it is to their presence so this is my best theory so far.
So let's headcanon that demigods have such a commanding presence or divine allure to them it makes people do double takes and makes them uneasy ( which may not always have something to do with their appearance but that can be a factor too)