What's the point of the prostitute character? She doesn't add much to the plot other than trying to seduce Mario multiple times.
BTW, why does she accuse Mario of taking advantage of her? Is she really a hooker or more a scam artist/extortionist with the nearby gang?
The film is aesthetically divided into two parts; the reality (Mario's side) and the ideal fantasy (Natalia's side). I saw the prostitute simply as a product of reality, it shows us that Mario's side of reality can become concrete where Natalia's idealistic dreamy world can't - there's nothing there. The other side of the town is full of other product of reality as well; the youth, the clients of the prostitute, the disco music etc.
I'm sure one could find much more from the characters, but simply it's a product of reality. Le notti bianche is all in all a very realistic film.
"I never said all actors are cattle; what I said was all actors should be treated like cattle"