Your question concerning The Great Gatsby is a little difficult to interpret. How is she so silly? Do you mean why? I'll assume you do and answer accordingly.
There are numerous answers to your question, depending on one's interpretation of Daisy. I believe Daisy has learned to appear silly and ditzy in order to get along. How else could she get along with Tom? He would not tolerate a woman thinking for herself.
The most important piece of evidence that Daisy just acts silly and ditzy is what she tells Nick in chapter one about her daughter. She was hoping for a boy but had a girl instead. She sarcastically tells Nick that she was glad it was a girl, and that she hoped she was a beautiful little fool, because that's what it takes for a woman to succeed.
The idea is that a woman's best hope for social and economic advancement in a society dominated by men is to marry a wealthy man, and the odds of doing that go up if one is a beautiful little fool.
Of course, that's what Daisy has done--pretended to be a beautiful little fool and "caught" a wealthy husband.
And that's why Daisy acts like a silly little fool.