We learn a lot about Curley's wife based on what other characters say of her. Thus, I agree with dstuva, the author doesn't make value statements but let's the characters display that.
The men call her a tart and a loo-loo. Not being common slang today, we infer by their use of these terms that they mean she is flirty, flighty and maybe a little permiscuous. Yet, her name, Curley's wife, suggests she is actually property. Always seeming to look for Curley, the men seem to wonder if she's not really either looking for Slim or looking for a man to be with other than Curley. This is a character I believe looking for an escape from her current life.
Another way characterization occurs besides other characters saying something about someone else is from a character saying something directly about themselves. Curley's wife admits in chapter 4 that she feels limited by Curley. In chapter 5 she explains that she could have been in pictures.
Curley's wife symbolizes the desire to be free from current situations. That is her significance.