Yeah, it felt kind of weird to have "believing in Santa" be a major plot point of a relatively straight-faced doujin. It wouldn't be out of place if the overall tone was more comedic or absurd, but drawing attention to it this much starts to strain the suspension of disbelief. If Nico can make something up about Santa being sick and standing in for him, and Maki will just accept it no problem, then it goes beyond Maki being pure, and into the realm of Maki being a four-year-old.
I don't actually care that much, though, because that page where Nico finds her present is everything forever. That one frame eclipses the rest of the doujin in feels. Except maybe for Maki learning that the keyboard was from Nico's childhood. That was also very sweet.