Definition of Mood
In spite of the novel's first-person narrator, tone and mood in a given scene often deviate. The tone Christopher uses to narrate the novel does not shift drastically, remaining largely pragmatic and straightforward throughout. Mood in the novel, however, ranges from hopeful to lighthearted to distressed depending on Christopher's circumstances and the reader's emotional reaction to said circumstances.
Note, for instance, the following moment of high emotion from Chapter 167. Christopher recalls a tense exchange between himself and his father, who reveals that he has deceived Christopher about his mother's death:
Father said, "We all make mistakes, Christopher. You, me, your mum, everyone. And sometimes they're really big mistakes. We're only human."
Then he held up his right hand and spread his fingers out in a fan.
But I screamed and pushed him backward so that he fell off the bed and onto the floor.
Christopher's narration in this passage is bare-boned: he presents his father's speech and their respective actions as straightforwardly as possible. A different narrator might utilize pathos or describe the pain and betrayal they feel using metaphor in order to generate a distressed mood. In this passage, the mood of anxiety and upset derives not from Christopher's description of his circumstances, but the circumstances themselves.
Through no fault of his own, Christopher often finds it difficult to communicate his emotions to neurotypical people. While his narration does not establish mood through conventional means, it is no less laden with feeling. Mood in The Curious Incident of the Dog in the Night-Time relies on diffuse emotion, built up over time throughout the text as readers increasingly empathize with its narrator.