The most conspicuous point about Curley's wife is that she does not have a name. The fact that she is nameless is significant, because this implies that she does not have an identity apart from Curley. In this sense, she is not significant at all. She counts for little, apart from trouble.
Second, Steinbeck portrays her in a negative light. In a conversation among the men, they confess that she is attractive, but she is dangerous. She has the eye, they say, which means that she is looking for companionship with other men. Because she is married to the boss's son, she can get the other men in trouble. The point is that men have to be careful of her. Here is the dialogue.
George cut the cards again and put out a solitaire lay, slowly and deliberately. “Purty?” he asked casually.
“Yeah. Purty . . . . but—”
George studied his cards. “But what?”
“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.”
Steinbeck also portrays her as tragic. She feels like she is in a dead end marriage. She says to Lennie that she could have been someone. She could have been in the movies, but all she has is her boring life now.