We could be here all day debating how beauty is subjective, and Milly isn't ugly ( she isn't hideous to be fair) etc. Believe it or not I have felt bad myself for saying she isn't pretty enough for the role.
Admittedly it's a bit shallow to judge her based on her looks, but unfortunately, or not, looks do matter in entertainment.
Objectively at the end of the day, numbers don't lie, and in the farms there's not need to sugar coat things to be nice. —She just isn't the type of pretty woman that makes woman jealous, and try to be her( they are critiquing her the hardest in fact); or move men me in droves to buy tickets just to see her, and endlessly simp for her like Sidney Sweeny, Gal Gadot or Emma Watson etc.
Poor casting overall.