The fact that Mrs. Phelps stays out of it makes her an important, but minor character in Matilda's life. Mrs. Phelps isn't ignoring Matilda, she just sees what the girl's up to and doesn't stop her. She even helps Matilda grow as a reader and nurtures her interest in books, and that's one of the things that help make Matilda strong. So in an indirect way, Mrs. Phelps is an awesome influence, giving Matilda the tools she needs to fight for herself.