How humour is injected and how is the novel made touching?

I love the line after Jem has lost his pants--Miss Maudie (I think) says Mr. Nathan has promised to shoot the next thing he sees in his collards, "be it dog, nigger, or--Jem Finch!"

Maudie says Stephanie Crawford once claimed to have seen Boo peeking into her bedroom window one night; she had asked if Stephanie had moved over in the bed to invite him in and said that shut Stephanie up.

But i thought Jem was quite traumatised when he received the camellias from mrs dubose?

Yes, he is furious at first and calls Mrs. Dubose a "hell-devil."

I think much of the humour comes from Scout as the narrator.

Scouts candid responses and observations creates humour. Her description of Judge Taylor as a "sleepy old shark' and Miss Caroline as "she looked and smelt like a peppermint drop" can be taken into consideration

Scout's inability to perceive things as a child is also very funny. When Miss Caroline asks Scout to hold out her hand, Scout thinks that she is going to spit in it. Her ignorance is clearly seen as she thinks Miss Caroline is "sealing oral contracts"

As mentioned, Dill contributes to the humour in the novel. Dill and Scout's plans to get married and have babies is an example of childish innocence.