Question: When Stacy is giving a speech at her bat mitzvah, she tells Andy Goldfarb that he's basic and not nice to people. What does she mean he's "basic"? And what parts of the film prove that he's not nice?
Answer: She's saying that even though he may be good-looking and popular, there is really nothing special or notable about him. His bad behavior was his meanness in driving a wedge between Stacy and Lydia, threatening their friendship.
Well, she told Andy that she destroyed her friendship with Lydia because of him. So, what other evidence is there in the film that he isn't nice?
Answer: He was very mean to Stacy by cheating on her with the popular girl, by calling her out on the ledge, and by making Lydia and Stacy drift apart.