Is the reader meant to feel sympathy for Curley’s Wife?

Initially, Curley's wife isn't portrayed in anything vaguely resembling a sympathetic light. She's a temptress and an unreconstructed racist, constantly threatening to have Crooks lynched if he doesn't show the respect she believes is her due as both a white woman and wife of the boss' son.
We're never sure of her age, but the derogatory references to her being "jailbait" clearly indicate that she must be quite young. Inevitably, then, Curley's wife is somewhat immature in some respects while still remaining wise beyond her years in others. As the only woman on the ranch, she has to figure out a way to get by in a man's world. And as ravishing beauty seems to be the only thing she has going for her, Curley's wife uses it to the full.
Like everyone else on the ranch, she has her own dreams, and this makes her more human, more sympathetic. Her desire to be a Hollywood movie star shows us something more beneath the cheap, trashy exterior. Tragically, like George and Lennie, she's destined never to fulfill her dreams, and we're left with a promise of what might have been.
We may not find Curley's wife the most likable of characters, but we can still have a grudging sympathy for her, even if we disapprove of the methods she uses to get on in life.