Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Moody
Because the whole tale is told from the perspective of Regan, a teenage girl, the tone can definitely take on her moody state of mind. Throughout the book Regan is trying to deal with living her own teenage life while hiding her sibling's big secret. Sometimes she doesn't handle it too well and ends up freaking out, at which point the book can take on a pretty despairing or self-pitying tone:
Why wonder? I almost asked. Adopt me. Give me a regular life, a happy childhood. Oops, too late. (15.16)
Even though things can get pretty melodramatic sometimes (for example when Regan says that she hates Luna, even though she obviously loves her to pieces), Regan still usually rebounds. The writing tone accurately reflects the turbulent ups and downs of adolescent life.