Fanny is a problematic protagonist because she is very passive and sometimes hard to approve of. Some readers find Fanny irritating and moralizing, other find her endearing. No matter how aggravating Fanny can be, she's also really sympathetic. It's really hard to not feel sorry for Fanny, even though she arguably causes some of her own problems. She's shy and others are often very mean to her. Mansfield Park is really Fanny's story and we root for her even as we sometimes get annoyed with her.