what is the mood of Maycomb

That really depends on when you mean. The mood in the beginning of the story was kind of nostalgic. Scout's stories of growing up, spending the long hot summer role playing and just being a kid were wonderful. There was a sense of contentment that felt calming and safe. As the novel progresses and he story of Tom Robinson becomes clearer, he mood changes. Maycomb has lost the innocence it once had. Indeed, Scout and Jem lose this innocence too. Their discovery of the disturbing details of the crime, the ugly racism that is cast about by their neighbours and the bigoted injustice of the jury thrusts our protagonists and the town into the sad realities of human nature.