Haven’t read the book, but watched a guy discuss the differences between The Devil Wears Prada and the movie.
His contention was that there were absolutely no redeeming traits about Miranda in the book and she had somehow failed upwards with no true talent. Andy the protagonist spends the whole time rebelling against the magazine and its people.
In the movie we see Miranda to be a horrid person but we see that overlays a keen eye and talent that has led her to the top. Moreover, Andy spends effort to fit in with the magazine people and she evolves as a character.