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.
In Steinbeck's Of Mice and Men, the author doesn't intrude and make value judgments about the characters: at least not that I can think of at the moment. Information about the characters is revealed, rather than directly stated.
For instance, in chapter three readers learn how cruel and ignorant and insecure Curley is when he attacks Lennie just because Lennie is smiling. Curley stupidly assumes Lennie is laughing at him, when in reality Lennie is smiling for reasons that have nothing to do with Curley. Curley is insecure and paranoid and violent. But these character traits are not directly stated by the narrator.
We also learn, again, how childlike Lennie is. He allows himself to be mercilessly beaten, because he is afraid that if he fights back George will be mad and won't let him tend the rabbits.
We also learn how dangerous Lennie is. He crushes Curley's hand and beats him down without even throwing a punch.