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.
It would appear Sanae was not the first to pull this trick
By the way, Cirno.Can you see them?The truth is, I just found these special clothes over in the forest of magic. Only idiots can see them.I see them!Yep, you really are a genius.