Why does it seem that Watanabe Mayu only has a good relationship with the weather in akb?

That's almost it, because they are all three-year students, and they are two trump cards for three-year students. Plus Ma You's character is not the kind of person who makes friends, so she really doesn't have any so-called friends in AKB. Everyone has a sense of existence like family. Even if she talks about the weather herself, they are not very good at talking.