Mithiwithi said: It's more that he's reusing a joke. Yuyuko's been doing topless scenes this entire manga (though from behind with sideboob rather than full frontal).
Which is another way to say, "Bomber Grape isn't being consistent at all."
I can easily add logic here as to why they didn't get tirn iff. There's a special Liquid armor that I've been secretly developing for 20 years. B.G. Just decided to use it with more than one character's clothes.