I don't think it's ironic that Mary herself is proud and vain. She says "that human nature is particularly prone to it, and there are very few of us who do not cherist a feeling of self-complacency". If Mary claimed that she herself was free from the failing, that would be ironic. As it is, it's just a moralising statement she probably got from a book. In another character's mouth, with slightly different wording, it may reveal self-knowledge.