Daisy has a number of functions in the novel. She represents a form of symbolic capital and also represents a certain insincerity of the upper classes. An object of romantic interest, she is also a person with her own romantic ideals and aspirations.
On a symbolic level, Daisy represents wealth. Her voice is "full of money" and she is the person that Gatsby and Tom each desire to attain (and/or keep) as a sign of their own achievement and wealth. She is, then, a symbolic property; a suggested sign of material prosperity. We can see this aspect of Daisy's character in a number of instances in the text, but none more telling than in Gatsby's specific ambition.
In this book, [Daisy] is the love interest of Jay Gatsby, who builds his mansion for her, and views her East Egg home from the point of its green light.
In order to be fulfilled in his quest for achievement/wealth, Gatsby feels he must marry Daisy. (This is also a matter of honor for Gatsby, in a way, as he feels that his initial tryst with her requires him to marry her according to the codes of chivalry.)
Daisy is more than just an object of desire, however. She is expressive of the basic insincerity and haughtiness of the rich. She cries over Gatsby's shirts in a scene that suggests she could have been happy with Gatsby and regrets marrying Tom.
“They’re such beautiful shirts,” she sobbed.… “It makes me sad because I’ve never seen such—such beautiful shirts before.”
Gatsby has turned out to be, by example of these shirts, rich enough for her.
Elsewhere, she proclaims that she is "sophisticated" in a moment of ironic and superficial self-mockery; a false and self-serving confession. This moment and others imply Daisy's concern with appearances above anything of substance.
Like Gatsby, Daisy is ultimately more interested in fulfilling the rather arbitrary expectations of the upper classes than she is in pursuing true romance, yet she entertains some latent dreams and aspirations for such things. She is wooed by Gatsby because of his money, it's true, but she is also interested in recapturing the idealism of her youth by running away with her first love.
The values that the novel brings into question are largely all found in the character of Daisy. She is sufficiently concerned with maintaining her lifestyle to allow Gatsby to take the blame for murder (though she claimed to love him). She is apparently entertained by her husband's affairs insofar as she seems to enjoy performing as the "hurt wife" while refusing to leave him (even when a suitor like Gatsby comes along).
Daisy is the object of desire but she is, critically, a quite superficial person. The desire for her that comes from Gatsby and Tom becomes superficial by extension. Ultimately, she is what her voice suggests - a symbol of upper class wealth and upper class society.