why are there so many pics of kaoru revealing her abdomen?

Because her slim waistline and just-been-laid hairstyle are her key points.

what does that have to do with her always being depicted lifting up her shirt?

Shirt up = waistline visible.

Because her slim waistline, just-been-laid hairstyle and her skirt lifting are her key points.

I thought she liked showing off her belly button because of that seen where she got her belly button kissed.