She’s a huge narcissist, it’s hard to like that. With the way she is introduced to the reader as a character her personality flaws make you super disappointed in her as a person, and betrayed by Eliot. She is introduced in juxtaposition to her brother, who is unlikable from the get go. That makes you as the reader predisposed to like her for disliking her brother, and everyone thinks she’s a pretty flower and so as the reader you do too. But then you learn that Rosamond is objectively worse since she’s not growing as a character, as opposed to Fred, who seems to show at least some initiative to be hard working, and you feel betrayed. You spend the rest of the time trying to find something redeeming about Rosamund for betraying your expectations because surely you werent fooled by a pretty face and manners as the reader.