She was definitely a good influence on Scout for several reasons. She provided a "mother" figure to Scout; without Calpurnia, she might not have had any mothering-except from the stern Aunt Alexandra later on. Secondly, Calpurnia is a good cook, a polite woman, has her head on straight, has good morals, and isn't afraid to let Scout know what is right and what is wrong. She disciplines Scout, chastises her, gives her love, and gives her attention. The best scene is during one of Alexandra's missionary circles; Calpurnia gives Scout strict and specific orders to behave, is aware of Scout's proclivity for mischief and boyish behavior, yet shows her trust and love at the same time.
Not only this, but Calpurnia opens up Scout's eyes to a different world. Having Cal around helps Scout to combat racism, see the other side of the argument, and understand the different social connundrums that exist in her world. Atticus trusts her completely, and knowing the kind of man Atticus is, that's a high recommendation. Cal's a great character, and Scout was lucky to have her.