Clarisse is almost entirely different from the Dowager Princess Clarisse in the books. Both do share two qualities: they are very strict and care about Mia.
The novel version of Clarisse is a smoker, manipulative and brutally critical of Mia mostly because of her love life, although she does comfort Mia at one point in the first novel by telling her a story about when she was a girl and had been picked on by a girl named Genevieve. According to Clarisse, Genevieve was just like Lana Weinberger (who was renamed Lana Thomas in the film), who used to torment Mia (until she had a change of heart in a later book). The film version of Clarisse is portrayed as a non-smoker, and is also the polar opposite of her book counterpart, being more nicer and more gentle.