Mrs. Van Daan is the mother of Peter Van Daan, the object of Anne's adolescent affections.
As her diary progresses, Anne expresses growing levels of frustration with Mrs. Van Daan, who belittles her son by referring to him as "The Thinker," which Anne feels is grossly unfair.
Mrs. Van Daan regularly admonishes Anne, stating that if Anne were her daughter, she would have been brought up to be very different to what she was. Anne's regular response to this is to express her gratitude that she is not Mrs. Van Daan's daughter.
Anne's diary makes it clear that she considers Mrs Van Daan to be a constant complainer—moody, panicky and neurotic. She is the source of many disturbances and much bickering in the secret annex which the Frank and Van Daan families are forced to share.
At first Anne Frank is warm to Mrs. Van Daan and the Van Daan family, saying they've become like one big family. However, as time goes on, things change. If the Diary had been fiction, the change for the worse would have been foreshadowed by the arrival of the Van Daan family a day early, with Mrs. Van Daan carrying her chamber pot in a hatbox, betokening stubbornness and an entrenched mode of thought.
If it had been been foreshadowing, it would have been exactly right. Mrs. Van Daan turned out to be selfish and highly opinionated and intrusive. Even though Mrs. Van Daan was hard working, she failed to make a lasting good impression by her continual know-it-all, advice-giving ways, a trait she shared with her husband, incidentally.
Anne Frank summed up her opinion of Mrs. Van Daan by saying,
The Van Daan motto: 'We get the best, we get it first, we get the most.' I've gained some insight into human nature since I came here, which is good, but I've had enough for the present.