Context would sure be apprenticed right about now.

I'm guess they're laughing at the fact that Rainbow's choice of pjs has a butt flap that has unfortunately for her, popped open while she was in such as pose that her ass was practically in everyone's faces.

Even poor Fluttershy is traumatized by that.

The bigger question however, is why the hell is Shining Armor in what appears to be an all female sleep over? Is it a R63 Shining?