Gatsby has elevated his imaginings of Daisy to impossible heights. So, objectively speaking, nowhere in the novel is she as great as she is in his mind.
Daisy does not figure much in Chapter III. In Chapter IV, Jordan shares her memories of Daisy with Nick. Jordan says she admired Daisy the most and Daisy was sought after (as a potential girlfriend, bride, etc.) by many young men. There must be something winsome about Daisy for her to have so many admirers, but part of her aura also comes from her looks and her social position.
Part of the reason, if not the main reason, that she marries Tom is for his money. We might consider this a flaw or it may be that she had been groomed and, therefore, pressured into marrying for such reasons. Daisy learns to love Tom and in the process, learned to let Gatsby (Gatz) go. So, she can not really be faulted for being pressured for marrying Tom and moving on. However, if there is truth to the notion that she married for money and therefore "sold out," she is not as flawless as Gatsby has imagined her.
In Chapter V, Nick notices subtle hints that Gatsby has some disappointment with his and Daisy's reunion. These lingering doubts don't prevent Gatsby from continuing to think of Daisy as the "golden girl." But Nick clearly perceives some doubt and this illustrates that she simply can not (through no fault of her own) live up to Gatsby's perfect vision of her:
As I went over to say goodbye I saw that the expression of bewilderment had come back into Gatsby's face, as though a faint doubt had occurred to him as to the quality of his present happiness. Almost five years! There must have been moments even that afternoon when Daisy tumbled short of his dreams--not through her own fault but because of the colossal vitality of his illusion.