Within the setting of Peal S. Buck's The Good Earth, the nineteenth century Chinese believed in the binding of women's feet; as a result of this binding of feet, women's "dainty" feet were only capable of tiny steps, steps that were considered very feminine and pretty. As Wang Lung goes to the great House of Hwang to purchase his wife, he regrets that he does not have enough money to buy a wife who has such feet. Instead, O-lan has what Wang feels are ugly feet. And, since her self-perception is derived from his evaluations, she, too, feels that her feet are ugly.
After Wang and his wife survive the Great Famine, they return to their home where Wang has productive years of farming. Shortly thereafter, he acquires more land thanks to the advice of his wife, he becomes wealthy. On his trips to the town he eventually frequents a tea house where the beautiful Lotus is, and Wang cannot control his desire for her. Wang demands two pearls from O-lan, which she has kept after finding them in the great house, so that he can buy this beautiful Lotus; O-lan does not wish to relinquish them, but she acknowledges that she is ugly and understands Wang's desires.
I think that the answer that you are looking for is that O-lan thought that she herself was ugly. She was, by all accounts, right.
She was picked as a wife for Wang Lung specfically because she was ugly. Wang Lung's father thought that it would be better for him to have an ugly wife because, among other things, she would surely still be a virgin.
Wang Lung is not happy about this and, once he gets rich, he becomes more and more dissatisfied with O-lan. So he takes a mistress and stops sleeping with O-lan.