What would you consider to be the overall mood and tone of To Kill a Mockingbird?

Like the previous answer, the mood changes. The first part
of the book is light and even funny at times. We see this side of the novel through the
personalities, perceptions, and actions of Jem, Dill, and Scout. What is particularly
amusing is the children's attempts to get Boo out of the house. Here is an
example:

Jem
gave a reasonable description of Boo: Boo was about six-and-a-half feet tall, judging
from his tracks; he dined on raw squirrels and any cats he could catch, that’s why his
hands were bloodstained—if you ate an animal raw, you could never wash the blood
off.

As the novel
progresses, the mood unalterably changes. When the trial of Tom Robinson is introduced,
everything changes. The mood becomes somber and serious. This should not be unexpected
as important themes are developed, such as rape, racism, courage, and
justice. 

In light of this, both Jem and Scout grow up and
shed their childhood. They have to face their community and life with a new set of eyes
where not all people are fair, not all people tell the truth, and not all people are
loving. Evil also even touches them, as Bob Ewell attacks
them. 

With this said, there is also a note of hope. The
children do not become bitter. They, like Atticus, will seek to do the right thing. In
the end, Scout shows this as she sees Boo as a mockingbird, who needs to be
protected.