What’s Up With the Title?
Novels that simply bear the name of a woman (or girl) are kind of an 18th- and 19th-century English thing: Moll Flanders, Emma, Jane Eyre, Tess of the D'Urbervilles. It doesn't matter if you've read these or not, you can tell by their names that they're old and about women.
Oh, and that there's going to be a lot of anxiety about sex. Women weren't supposed to have sex, and weren't even supposed to want to have sex. If they did have sex, it had to be with their husbands, and they had to be not that into it. All of the novels mentioned above begin with single women who are, therefore, in sexually treacherous waters. In the context of their time period(s), even naming a woman in a title is kind of scandalous. It's like putting her up on a billboard in her nightie. By giving the novel Daisy's name, James is referencing a literary tradition of young, single women for whom attractiveness and publicity spelled danger—and sometimes death.
We also get a lot of information about Daisy simply from her name. "Miller" as a surname implies she's new-money; though she may have fancy clothes, underneath she's a simple girl from upstate. Millers (like those who have a saw mill or a wind mill for grinding corn) are not typically from illustrious backgrounds.
If you were writing a novel about a rich girl, her name would probably be Vanderbilt or Van Der Woodsen, not Miller, right? And if you were giving flowers to said rich girl, would you buy her a bouquet of daisies? Or would you go with something more along the lines of roses or gardenias or calla lilies? That's what we thought. Daisy Miller. She sounds like someone you'd meet on a bus, and that's sort of the point.