What is the effect of James's many references to Daisy's prettiness in so few pages?

There are a number of references to Daisy's prettiness in the span of a few pages. The use of such a literary maneuver impresses upon us Winterbourne's first distinct feelings about Daisy. He thinks of her as innocence personified; to him, Daisy embodies a virginal wholesomeness that is set off to perfection by her fresh, wholesome beauty.
The effect of so many references to Daisy's prettiness is that our attention is drawn to her physical attractiveness ad nauseam. The effect is simultaneously cloying and intriguing.

The young girl's eyes were singularly honest and fresh. They were wonderfully pretty eyes; and, indeed, Winterbourne had not seen for a long time anything prettier than his fair countrywoman's various features—her complexion, her nose, her ears, her teeth.

James's use of repetition causes us to question his reasons for doing so, and he doesn't disappoint in providing the answer. In the story, he demonstrates the difference between American and European views of female virtue. In noticing Daisy's fresh beauty and all-embracing openness, Winterbourne becomes filled with questions about American female honor.
He questions whether Daisy's outward freshness hides the taint of promiscuity:

He thought it very possible that Master Randolph's sister was a coquette. . . Certainly she was very charming, but how deucedly sociable! Was she simply a pretty girl from New York State? Were they all like that, the pretty girls who had a good deal of gentlemen's society? Or was she also a designing, an audacious, an unscrupulous young person?

The European view of overly-friendly young women is that they must be "coquettes," whose only aim is to ensnare men in their debauchery. Ultimately, Winterbourne concludes that Daisy doesn't exactly fit the picture of an ideal "coquette." She is pretty, but she is also unassuming and chaste in her conduct. In the end, he decides that Daisy is just a "pretty American flirt," not one of those "dangerous, terrible women."
The repetitive description of Daisy's prettiness is a literary device that highlights the theme of differing European and American prejudices regarding the outsider, female virtue, and class hierarchies.