So many people dislike fanny, because we all on some level relate to the selfishness, materialism and vanity of the Crawfords, and the sisters. Throughout the novel, she condemns their behavior, and through this, we as the reader feel that fanny is also reprimanding us. it makes us aware of our own moral shortcomings, and this makes us uncomfortable. and instead of praising Fanny for being strong and always sticking to her morals no matter what, we call her weak - so we don't have to address our own faults and look into ourselves.