In The Great Gatsby, what does Daisy mean when she says, "I'm glad it's a girl. And I hope she'll be a fool—that's the best thing a girl can be in this world, a beautiful little fool"?
Daisy is obviously unhappy and is expressing her disillusionment when she expresses this sentiment. Her remark comes after she had informed Nick about her cynicism about everything. Prior to this she had been speaking to Nick and he noticed a peculiar change in her, as he states:
...her voice compelled me forward breathlessly as I listened — then the glow faded, each light deserting her with lingering regret, like children leaving a pleasant street at dusk.
It seemed as if the more Daisy spoke about her life, the more aware she became about how empty and meaningless it actually was. The interruption later, when Tom received a phone call from his mistress, clearly added to her disappointment. She tried her best to hide what she felt, but Nick had noticed a definite change in the atmosphere and perceived a tenseness, especially after the phone rang again. The tension was so thick that Nick felt the urge to phone for the police.
Daisy later informed him about her misery when she said:
“Well, I’ve had a very bad time, Nick, and I’m pretty cynical about everything.”
Nick believed that she had more than enough reason to be so negative but she did not offer any more insight. He then changed the topic and started talking about Daisy's child. She declared that Tom had not even been present at his child's birth and that she, after determining the child's gender, made the remark quoted in the question.
Daisy's remark is a reflection of what she believes a woman should be like since she realises that she had been made a fool of. Tom has had numerous affairs and has not shown her the care and affection she believes she deserved. It would have been better if she were stupid enough not to care about what her husband did. As far as being beautiful, Daisy is suggesting that a woman who is attractive would be able to hook any man, as she apparently did when she drew Tom's interest and married him.
It is quite ironic that Daisy should find herself in this situation for, just as Jay Gatsby discovers later, the dream is far different to the reality. Daisy could not wait for Jay and decided to marry Tom, probably believing that his wealth would be enough to ensure happiness. Jay, similarly, believed that wealth would secure Daisy and that he would be able to recreate the past. Tragically, what he had was not enough and the dream was never fulfilled.
Further irony also lies in the fact that Jay could have given Daisy the love and attention she so desperately sought but she was not prepared to give up her family and life of privilege. She told Jay that he wanted too much and accepted her lot.