In this context, Daisy suggests that she hopes her daughter is a fool. Daisy expresses a defeatist attitude. She implies that women have a difficult role to play in the world, so the best chance for her daughter to be happy is to accept the traditional role of subservient woman. Essentially, in this case, Daisy is hoping her daughter is ignorant because ignorance is bliss. If her daughter is unintelligent, she won't know how the world really works and/or that there are alternatives to marrying people like Tom.
It is important to note that when her daughter is born, Tom is not there.
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.
Later in the novel, Daisy reveals that Tom has had affairs (in addition to his affair with Myrtle Wilson). We, readers, can only speculate as to where Tom is at the time of his daughter's birth, but even if he isn't off having an affair, he is still negligent for not being there.
Instead of hoping that her daughter will be self-reliant and able to make informed decisions (and hopefully avoiding marrying someone like Tom), Daisy feels defeated and pathetically hopes that her daughter will be happy in her ignorance.