I'm somewhat biased since a girls stomach is one of my favorite things about her. I love love love a girl with a toned and flat tummy. That being said you still have to be classy about it and not slutty. (see below):
Yes having shirts with vulgar sayings and having your bra and panties sticking out is not hot, especially if its obnoxious colors like hot pink or leopard print. That kind of stuff is only hot when seen in the bedroom.
I have a pretty toned stomach and I have a natural tan most of the year. I love days when I tie my hair back in a pony, put on a baseball hat with my pony through the back, a pair of sweatpants, sneakers, and a short sleeve baggy crop that shows my bellybutton. Errand day. No make up. It's a playful look, not trashy.