Here's a novel theory:

But I can’t help but remember how her cheerfulness may just a coping mechanism to deal with her weaknesses, her fears, her pain. The greater the latter, the more outlandish her genki behaviour. I haven’t seen the manga but Minori strikes me as someone who’s in a lot of pain.

That such thinking came about is not surprising. Of all characters in Toradora, Minori is the least explained, and so she has an appearance of unnnatural behaviour. Most likely though, it's just the creators not giving everyone enough attention.

I should note that similar ideas were advanced about Tomo Takino, by fanfic writers. The imagination abhors the vacuum.