I actually made an argument on YouTube about this.
It was in response to a specific video, so bear with the directed comments.
I view her as more of those people who "doesn't grow up", not an idiot. She also has her own way of thinking, according to my analysis.
See, the way I see it is that she's definitely NOT an idiot, but she's about normal in intelligence. However, being that she's (game age + chronology since game was released) like 80 or something, and youkai can live for hundreds or thousands of years (if not indefinitely, as some other commenters pointed out), she's *considered* a child, but is in fact an adult. She's a tsundere and a bit of a ditz sometimes, as well.
Her personality isn't because she's "growing" as you say. More-so, it's because she's like any 20 y/o otaku or geek you'll meet, only not with games, anime, etc. She has the mentality of someone who just "never grew up". I know people like that; they still act like they're in their pre-teens, even though they're definitely not. She also, as a tsundere, is actually searching for approval from someone. By being strong, she expects that someone will notice her, and she wants to impress people. She basically just wants to be liked, and she's a bit awkward, so this is how she does it, which in turn makes her come off as acting "stupid".
Regarding the shaved ice you mention, I don't think that's *either* stupid *or* smart; it's just, as I said earlier, ditzy. Despite her powers and physical strength, she's still a fairy, which is like an elemental (in her case, a Cool Elemental, and she acts on water much of the time, since you can freeze water), so to her, she wasn't eating shaved ice or popsicles for a sweet flavor; she liked them because they're cold.