Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Fluid, Ambiguous
Sort of feel like you can't tell what the narrator thinks about Artemis or… well… anyone in the book for that matter? We noticed that too. But since this whole book is supposed to be a case file, this is exactly as it should be. There shouldn't be any obvious favoritism or admiration for any particular player in the story, because case files are—in theory—objective and devoid of feeling.
But we're not calling the tone of this book objective, now are we? Nope—we're calling it ambiguous and fluid. This is because there are plenty of times when characters are alone, doing some good old thinking and feeling, and when this happens, the tone toward whatever's happening becomes the tone of whatever character we're hanging out with. And since two of our main characters are about as different as night and day, this means the tone definitely shifts around throughout the book—there's no one set tone taken.
Adding to the ambiguity is the fact that ever now and then a disembodied voice breaks through a character's reflections to undermine their self-impressions. For example, Mulch is described in a pretty basic way up until the narrator (who really doesn't like Mulch much, evidently) breaks in and makes a self-righteous, editorializing comment:
As if this account didn't already suffer from an overdose of amoral individuals. (7.1)
It's a super subtle manipulation, but it's there nonetheless, and it can catch a reader off-guard and influence their opinions. No matter how much you might have liked Mulch, suddenly you also have to think about him as a criminal, "amoral individual"—just like that, the tone has shifted again.