It's possible that:
* Her parents gave her the name. This is the most likely scenario, but the story doesn't explicitly state it.
* It was a nickname. Maybe her real name was something else entirely, and "Cinderella" was a nickname given to her by her stepfamily or other people.
* It was a name she gave herself. This is less likely, but not impossible. Perhaps she chose the name as a way to define herself after being mistreated.
The important thing is that the name reflects her low status and the hardships she faces, making her a relatable character.