That’s a really important question.
One of the challenges of reading a novel that’s written in the first person is that you have to decide how much to trust the narrator. In Catcher in the Rye, for instance, Holden Caulfield shows you over and over again that he is an inveterate liar, but for some reason you still kind of suspect that he is telling you the truth. In other novels (American Psycho comes to mind), the narrator is clearly unreliable.
In Alaska, I think Pudge is trying his best to be accurate to his experience and memory, but it’s also clear he is writing all this down at some point in the future. From the structure of the novel and from a few moments of foreshadowing, I think it’s pretty clear by the end of the book that he knew about Alaska’s death before he started telling the story. And when you look back at the dead, I think they are inevitably more beautiful. Plus, you’re absolutely right that when you’re romantically enthralled with someone, you see that person as more beautiful than other people might. So I think Pudge’s descriptions of her beauty are probably shaped by his memory and his experience. (And while some other people—Takumi and Jake for instance—also find her physically attractive, the Weekday Warriors never express much physical attraction to her.)