Compare the differences in the description between when we first see Curley's wife and when we last see her.
Steinbeck clearly describes Curley's wife in two distinct ways when we first see her in chapter 2 and when we last do in chapter 5. In the initial descriptions, she is shown to be flirtatious, one whose sexual energy overcomes Lennie and George. Her physical appearance, in terms of how a woman is supposed to look to a man, is accentuated:
A girl was standing there looking in. She had full, rouged lips and widespaced eyes, heavily made up. Her fingernails were red. Her hair hung in little rolled clusters, like sausages. She wore a cotton house dress and red mules, on the insteps of which were little bouquets of red ostrich feathers.
Such details are accentuated with Curley's wife's "arched back" and the "twitch" in her body. These details of sensuality can be immediately contrasted with the last time we see her, one in which she is dead. It is at this time the sensuality has been replaced by a beauty brought about by innocence and purity:
Curley’s wife lay with a half-covering of yellow hay. And the meanness and the plannings and the discontent and the ache for attention were all gone from her face. She was very pretty and simple, and her face was sweet and young. Now her rouged cheeks and her reddened lips made her seem alive and sleeping very lightly. The curls, tiny little sausages, were spread on the hay behind her head, and her lips were parted.
What was socially constructed notions of sensual beauty has not become elevated to a point where Curley's wife is "pretty and simple." The difference in her described appearance might be Steinbeck's perception that what is embodies fundamentally different elements between social acceptance and a realm that exists outside of it. The beauty that Curley's wife holds is symbolic of this difference.