"Pretty" is definitely a lower degree of beauty than "beautiful". And so it translates to "ładny".
piekny is translated as pretty here
amongst thousands of other examples
When I read this sentence I think that a woman asks the question, even though is "piękny" and not "piękna". Is it so?
A woman would say "piękna".
A person with a complicated gender identity could say "piękny", but then it means they use masculine forms to describe themselves.
Handsome is "przystojny".
So it is rather unusual that the default Polish phrase here is said from the point of view of a man...
If it would be a noun phrase (piękna kobieta), then it would go into Instrumental (piękną kobietą). But an adjective will stay in Nominative.