After Lennie accidentally kills Curely’s wife and her body is alone in the barn, Steinbeck describes her thusly:
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.
When he reader is first introduced to Curley’s wife at the beginning of the novella, Steinbeck paints a picture of a sexualized, overly made up “tart,” as the men on the ranch repeatedly call her. In death, her physical features are identical (red cheeks, red lips, sausage curls). However, the meaning of these features changes because her expression no longer conveys the unhappiness within. While I don’t necessarily think Steinbeck makes her “prettier,” as your question implies, his description does make her more sympathetic.
This passage evokes pity for Curley’s wife, who up to this point has not been a very likable character. I think Steinbeck does this because he is trying to lead the reader to interpret her differently than the other characters in the text. For the majority of the text, she is only portrayed in a negative light through other characters’ attitudes. This description of her lifeless body suggests another reading contrary to the prevailing attitude of the men on the ranch. This is what makes what happens as a result of her death such a tragedy; if Curley’s wife was a one-sided villain, readers would not be able to sympathize with her as a victim. Clearly, Steinbeck wants his readers to understand that she is another victim in this text.