A mood of a novel, or any text, is derived through the author's use of language and imagery. Therefore, a mood is felt by the reader.
The mood of Walter Dean Myers' novel Monster, therefore, can be different for any given reader. Some readers may feel put off by the material, the protagonist, or (really) anything from a text. Sometimes, regardless of how hard the author tries, the desired mood is unreachable.
As for my own personal interpretation of the mood, I would tend to lean towards the mood as one which is anxious. Steve Harmon is on trial for murder. His story is one that is far too common in society today. As a teacher, I feel anxious for Steve, the situation he is in (jail and the trial), and about the verdict.