I like Melanie Mayron, but tbh she has always looked like a person with an eating disorder, especially in Thirtysomething and her vulnerability made it hard for me to take her seriously as a actress with more depth.
I thought the portrayal of Marianne seemed a bit 2D.
Yes the character is based on a real person whom Fenelon befriended on the train and later prostituted herself to Nazi soldiers for food. But like many things about the movie that differ greatly from the book, the real life "Marianne" was not thin. Fenelon described her as unfortunate looking, with a large body but smaller legs. Supposedly the soldiers liked her because she had more meat on her. Fenelon had a lot of unflattering things to say about this woman, but she does credit her for saving her life. When they first arrived at Auschwitz, Fenelon was going to join a mother and child and ride an ambulance with them, and Marianne encouraged her to walk instead, saying they've been on a train for days and could use the exercise. The ambulance went straight to the gas chambers.