The entire movie, Mahoney plays the 'air piano' and music plays. The audience assumes it's just a directorial quirk, until the end, where Mahoney realizes she does possess magic. The magic was there the entire time, but we didn't notice.
On top of that, it's cleverly set up earlier. The term 'sparkle' is used as a metaphor for the magic inside Mahoney, and in the scene where she asks Mutant about it, the dialogue goes like, "You've got that thing you do with your hands." "That's a quirk." "Quirk's not a sparkle?" The film's screenplay just became even more brilliant.
Given this store is literally magic and actively displays this nature, how come this store is not an internationally known tourist attraction with a queue of kids and tourists several miles long every day. That's because it's arguably a metaphor for all of the little miracles that happen all around us that many people don't notice. Not just that, but the store is also a metaphor for wonder. Besides Mahoney and Mr. Magorium (and Henry at the very end of the film), no adult can see how amazing the store is. Not even the woman with an octopus on her head. She looks really apathetic in the store before opening the picture book and even with the octopus on her head, she just looks annoyed and disturbed. Adults don't have the sense of wonder children do, so they have a Weirdness Censor that stops them from seeing what the store truly is.
If there are any other adults who can see the wonder, they wouldn't be able to tell anyone else about it anyway.
