What humor does Lee use to lighten Atticus' discussion with Jem about reading to Mrs. Dubose?

THere is a little bit of humour here. Mrs. Dubose asks Jem to read to her for a month to make up for the cut flowers. When Jem returns home, the following exchange takes place,

"Atticus, it’s all right on the sidewalk but inside it’s—it’s all dark and creepy. There’s shadows and things on the ceiling...”

Atticus smiled grimly. “That should appeal to your imagination. Just pretend you’re inside the Radley house.”

After all that has happened this is a little funny.