As the the story is told from Meursault's point of view, it is easier to understand why he likes Marie than she does him. However, we can infer reasons for her attraction. Marie, in fact, likes Meursault for many of the same reasons he likes her.
First, they both enjoy sex. Meursault tells us this is the case with him. In Marie, we can infer it from such gestures as her desire to be kissed by him, her rubbing against him, and her wrapping her legs around him as they swim in the sea.
If Marie is uncomplicated to Meursault, he is the same for her, which she likes. She enjoys spending time with him, and she also likes him because he is "different"—which is also one of the reasons Meursault likes Raymond. She seems to appreciate that Meursault is honest with her: when she wants to get married, he tells her he doesn't love her, but he is willing to go along with it all the same. This seems to be fine with her, as she wants the practical and concrete things in life, such as marriage, and simple pleasures like swimming in the sea, food, and sex. She also likes the space to do her own thing, such as visiting her aunt. As Meursault is largely indifferent about her, as he is about everything else, she has the freedom to come and go as she pleases without having to deal with a controlling partner.
Meursault likes to be around people who are easy and uncomplicated to be with, and Marie seems to share this trait. For Marie, the relationship is convenient and meets her immediate, surface-level needs.