Heck yes. Very attractive! Why not? I think most young girls nowadays actually wear flip-flops with shorts or jean skirts all the time when it's warm. I love them because it gives their legs (considering they're hot! ) natural curves and shape that's very sexy to me. Personally, I don't think heels goes with shorts except as modeling or being in some beauty contest. But if she wants to accent her legs for whatever reason she has in mind, of course heels would go beautifully with shorts because they accent the legs so much. Heels automatically convert however the girl is dressed to being "dressed up"! If the girl has pretty legs either is attractive to me.
I think I speak for most men when I say that we don't really pay attention to when you're supposed to be wearing flats vs. Other shoes. If you're cute and in good shape, the minute fashion combination details are pretty much lost on us. Sure, having good style overall is important, but I wouldn't have any idea that you weren't "supposed" to be wearing flats with shorts.