Curley's wife is an interesting character in the book. We cannot reduce her by saying she is a good or bad character. Things are more complex.
First, we should say that she is not even given a name. This says something about the status of women in the book. In essence she does not count for much. When she is introduced, the book portrays her as temptress. Here is a quote:
“Well—she got the eye.”
“Yeah? Married two weeks and got the eye? Maybe that’s why Curley’s
pants is full of ants.”
“I seen her give Slim the eye. Slim’s a jerkline skinner. Hell of a nice fella.
Slim don’t need to wear no high-heeled boots on a grain team. I seen her give Slim the eye. Curley never seen it. An’ I seen her give Carlson the eye.”
Second, Curley's wife, perhaps more than anything, is lonely. She is alone on the farm. She barely has anyone to talk to, and she is in a marriage that clearly does not make her happy, even if we do not know how she and Curley got together. The only person that talks with her is Lennie, which does not end well. In many ways, she, like Crooks and Candy, feels alienated.
In light of these point, I would say she is not bad or good, but sad. She is a product of a world that is alienated and broken.