Quick, name two places in which appearances are really, really important. If you said "7th grade" and "the art world," give yourself a gold star, because now you're thinking like a Danish YA author (in which case maybe you should get your Dannebrog on and give yourself a red star with a white cross.) In Nothing, we see that adults are fronting just as much as kids are, but adults are doing it for money. Case in point: the newspaper reporter who at first denounces the heap of meaning, then after the MOMA deems it worth millions, says he didn't recognize the art at first because he only saw it from one side.
Being an adult just means you've had more time to develop your masquerade. You become less authentic the older you get.
When Agnes sees Jon-Johan crying before Sofie cuts off his finger, she realizes she prefers who he pretends to be to who he really is.