I have read the book and seen the movie and I am puzzled.
We all know it is difficult to condense a book into a move. You obviously need to drop or change some subplots or events to fit everything in the 2.5 hour run time and obviously you need to make the characters more likeable so you will follow them into the story.
However some of the changes either made the story make no sense due to being under developed or because they misunderstood the underlying symbolism. That or they have a character straight up explain the plot to you as they don't have the time / skill to communicate this to the audience through the scenes.
Another issue is that the girls are treated as best friends who sit together, talk together and therefore must have common interests or traits. This confuses the story because neither girl is seen as truly good or evil, they're both in the middle.
The whole drive of the story rest on two points:
One the girls by personality and appearance are supposed to be complete opposites. One a gloomy recluse who delights in the macabre (but has a kind heart and is truly good at her core) and the other is a beautiful social butterfly who does good deeds (but everything is based in selfishness or cruelty).
And the other is Tedros. Sophie wants him as she believes she deserves a prince and he's the best one. Agatha wants him to kiss Sophie so they can go home but eventually they fall for each other.
Neither of these were suitably conveyed in the story.
This would have been better as a short series where more time could have been given to flesh out the story.