Although the whole theme of Uncle Tom's Cabin is that slavery is an evil institution that should be abolished, and though it caused a huge upsurge in abolitionist sentiments after it was published, abolitionists don't play too prominent a role in the novel. Stowe is ambivalent about them because they condemn slavery without actually living near blacks; they might want the slaves to be free, but they don't necessarily like or want to be around them.
Miss Ophelia, Augustine's cousin who comes to help organize the St. Clare household, is perhaps the chief abolitionist in the novel. She comes from New England and abhors slavery, yet she is prejudiced against slaves. Although she wants to help and do good for Topsy, she can't love her because of her black skin. Topsy senses the lack of love, and this makes it hard for Ophelia to have a positive influence on her young charge. As Augustine says, whites in the North condemn slaves, but they themselves often don't like them.
Ophelia nevertheless has a change of heart after Eva's death and is honest with Topsy about her struggle to love her. But as she begins to love the young slave, Topsy, who has been hardened by abuse, begins to respond.
This connects to postcolonialism because many people were in favor of freeing places like India or countries in Africa from imperialism while at the same time lacking love or understanding for the native peoples there. Many Britons, for example, supported liberating India but were unhappy when Indians began moving to their country. It is easy to love the "other" at a distance and harder when it is up close and personal.