Cirno is obligatory comedic relief. Maybe a little ditzy, perhaps a bit of an idiot, but that doesn't mean she's like, retarded to the point of having no common sense or something. She's a fairy. She happens to have the ability to manipulate the cold, and to form ice. Ice can be pretty potent - it's dense, if it breaks and falls apart, all you have to do is melt it down and then freeze it again and it's just as strong as it was before... science science science Touhou logic magic magic, Cirno just happens to have been lucky enough to have gotten an ability that can actually be pretty powerful. Ergo, she's a fairly high-key character - except she's not a full human or full youkai, so, short straw.
