Why is Alison unlikeable?

One of ther reasons that Alison is unlikeable is her inability to take a stance. She married Jimmy partly as a rebellion against the proper, predictable, stultifying precepts of her class, even though she, herself, continues to embrace the lifestyle in which she was raised. She is emotionally detached.... stiff and uncommitted to her marriage.

When Helena arranges for Alison to leave her husband, she doesn't hesitate and easily returns to her prior life.... her parents, their values, and everything that she protested to disapprove of. She later returns to Jimmy after suffering a miscarriage, and it is at this point we begin to feel genuine empathy for her circumstances. Jimmy's cruelty and nastiness far outweigh Alison's weaknesses.