Are you sure it's "only dummies *can* see it" (emphasis mine)? It makes more sense if Marisa says dummies *can't*, and Cirno claims she can see it so she doesn't look like a moron, Especially given the last panel.

But the point of Cirno strips is to prove that Cirno is a moron. I think it makes more sense the original way, given the fact that Cirno thought about it and still got it wrong.

I think Marisa's plan was to give Cirno an opportunity to show she isn't an idiot. Since our dear Cirno is so awesome, she manages to foil Marisa's plans.

In short; The Emperor's Mind Game.

It would appear Sanae was not the first to pull this trick