Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Humorous, Honest
We're reading Georgia's diary, which means the tone of this book is honest through and through. She isn't trying to impress anyone or thinking about getting published—the pages of this book are where Georgia recounts her trials and triumphs as they happen, in all their glory. Fun fact: Books that are written as letters or diaries (or other similar documents) are called epistolary novels. This is most definitely an example.
Luckily for us, Georgia has a fondness for humor. So even when she's being a bit serious, her observations are often still tinged with humor. For instance, upon hearing her parents arguing Georgia writes: "Heard Mutti and Vati arguing. Oh perfect, now they'll split up and they'll both want custody of me" (4.486). Note the sarcasm in there? "Oh perfect" just oozes with it, and in doing so, takes what might be a heavy moment—parents arguing can be kind of ominous—and gives it a bit of lightness. The humorous tone here adds to the honesty, subtly letting us know that this too shall pass.