[ You have the pleasure to be looking at Chiaki Nanami - at least it's not the close-up shot of her face! ]

One of the basics of humor theory is that we perceive as funny what is unfitting or unexpected. So the idea of setting up a formal dress party and then playing this is probably hilarious to her. Her sense of humor seems to be twisted but rather basic.

Though of course, this is also why humor is so closely linked to offense. What is funny is societally determined and in the old days, it often went on the cost of others. Seems we're the others now.