There's no getting around the fact that Crank isn't exactly a happy story. Where Kristina starts out with talent and promise as a student, she trades her entire identity in for a dance with the monster. Appropriately, the book's descriptions and word choice mirror this, particularly when Kristina reflects on what's been lost in chasing after crank.
The book's dark tone is pretty much summed up in "Problem Number Four: Feeling Good," where Kristina presents an exhaustive list of things that typically bring joy to normal people, including nature, riding in fast cars, and personal successes in school and music. She then reveals the core of her problem with drugs: "Somewhere on my stroll with the monster, I'd lost these things" (6-7). She knows little to no joy, and since she's our narrator, the tone follows suit.
Crank sucks the happiness out of her life, leaving her mood dependent on how much of the drug is in her system. The tone of this statement reflects this fact: "After awhile," she says, "even high, I could almost make believe food didn't taste like cardboard, almost float down into REM sleep, almost function the next day, almost look forward to my 17th birthday" (Feeling Good.6-9). It becomes clear that Kristina's aware of what she's sacrificed to get high—and that the decision to do so definitely wasn't worth it. All those almosts are just depressing.