Does lula speak for the majority opinion in the congregation? why does the author include her in the church scene?

It could be stated that she seems to be "bitter" about life, she does not like Calpurnia bringing those white kids to their place, their church. She might believe that "segregation should be a two way street" and the congregations seems not to agree with her.

She was probably placed in that scene in order to contrast their way of thinking about segregation.