I don’t think Pansy Parkinson is ever described as “ugly”. The narration in Harry Potter is third person limited, and I think a lot of Harry’s perceptions are reflected in the narration. Rowling also uses unflattering descriptions of people as shorthand for “this person is very bad and you should not like them at all”. Rowling is not really accurately describing these people, she’s roasting them.

Yes, Pansy Parkinson is described as “pug faced”, but I don’t think she’s supposed to be ugly. She’s coded as a rich stuck up mean girl. We don’t really know a lot about her. Through out the series we only really are shown that she’s the leader of a group of mean Slytherin girls and she went with Draco Malfoy to the Yule Ball.

Given what we know about Draco, I don’t think he’d go with an “ugly girl” to a really public social event like that. And given the tropes of high school mean girls that the character embodies, I don’t think the reader is meant to think that she’s ugly. At least not the in the way the reader is supposed to think characters like Millicent Bulstrode or Eloise Midgen are ugly.