I guess the joke is that her underwear has a grey-white checkered pattern, which is the basic see-through "color" (working with layers) in most digital painting programs.

I guess I could translate the whole thing, but I'm a lazy ass.. so:

The two other girls are scocked by how perverse her undies are, while she just thinks its rather plain. (Jeanne and Hime must have 4th wall powers)

I wonder if a bra like that actually exists. Or maybe a swimsuit