Near the end of Chapter One, Daisy is telling Nick how she has become very cynical. When Nick tries to lighten the subject by switching topics to her daughter, Daisy responds by saying:
It’ll show you how I’ve gotten to feel about—things. Well, she was less than an hour old and Tom was God knows where. I woke up out of the ether with an utterly abandoned feeling, and asked the nurse right away if it was a boy or a girl. She told me it was a girl, and so I turned my head away and wept. ‘all right,’ I said, ‘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.'
Note that this is in the context of Daisy's comments about how cynical she has become. Nick had already noted "turbulent emotions" in her which is why he tried to lighten the mood by talking about the daughter. Daisy says that she woke up in the hospital or delivery room and Tom was not there. He essentially abandoned her; a habit that would continue throughout their marriage. Upon learning that the child was a girl, Daisy began to cry. She may have felt that her daughter would have a similar fate; that she would grow up, marry a brute like Tom who cheats on her, and be pressured to simply accept this role. Daisy seems to indicate that if she'd had a boy, she wouldn't necessarily be happier about that but she would feel better knowing that the boy, in a man's world, would have an easier life than a girl.
Daisy initially falls back in love with Gatsby because there was genuine love there and because he offers a kind of escape, even if it was only a fling (or "spree" as Tom calls it in the novel). When her daughter is born, Daisy is worried that she (Pammy, the daughter) will have to marry someone like Tom and end up frustrated and cynical, just as Daisy has. Daisy's cynicism is legitimate. In Chapter 7, Tom treats Daisy like a child, being condescending even as he admits his infidelities:
She does, though. The trouble is that sometimes she gets foolish ideas in her head and doesn’t know what she’s doing.” He nodded sagely. “And what’s more, I love Daisy too. Once in a while I go off on a spree and make a fool of myself, but I always come back, and in my heart I love her all the time.
Daisy mockingly but legitimately says she hopes her daughter will be a beautiful fool. Being beautiful, she will have more suitors, more options of men to marry. Being a fool, she will be too ignorant to realize if she's married a cheating brute of a man. This is the same as saying "ignorance is bliss" - being too foolish to know that her life is not so wonderful.
Daisy is angry that Tom is not there for the birth and she is relieved her child is a girl. She's happy she is a girl because she wants her to be a pretty little fool. Daisy says the best thing you can be in this world is a pretty little fool.