Well, she did already say that even if she does summon something, Reimu can just take care of it. Which of course from a narrative perspective means that whatever she ends up ultimately summoning will probably be way out of Reimu's league.
I figure she's just spoiled into extreme naivete by Gensokyo's current golden age, and possibly her own powers clouding her judgement or affecting her personality. Honestly, personality-wise, she seems more like a Touhou youkai than a human to me since she's so... capricious and not-serious. Sanae at least has the excuse of being from the outside world. I suppose to put it into perspective, Sanae's an anime fan who entered a world of anime while Kosuzu's an anime fan BORN into a world of anime. Assuming she's from the latest generation of Gensokyo after the spell card rules have been long established, I think Kosuzu's used to youkai who only play evil instead of actually are evil along with a shrine maiden that can just solve everything anyways, and thus it doesn't dawn on her that something seriously bad could happen.