The meaning of "To kill a mockingbird" is to kill innocence. As stated in the book, "Remember, it is a sin to kill a mockingbird'.
The tone of "to kill a mockingbird" changes throughout the story. It can go from being light-hearted & humurous to childhood fear. However, the overall tone is that of hope and promise for a better future in the world.
Metaphors, Similes, Personification, etc, are all used in "To kill a mockingbird".
Metaphor: For example, "it is a sin to kill a mockingbird", because it does you no harm. (comparison of killing a mockingbird with the mockingbird itself)
Simile: "He was as good as his worst performance."
Personification: "it is a sin to kill a mockingbird" (personification of the mockingbird as a person)
these are just a few
hope this helps