Yeah, I sometimes don't get Team Ninja's design choices at all. Kasumi always had pretty long hair. Her long and thick high ponytail suggests exactly that. But then they give her that short and very thin open hair. One might now say: "Oh, it's not that bad. It's probably the best they can do without too much clipping and without straining the graphics engine." But then again, just look at Kokoro and Nyotengu. Long, thick hair without hardly any issues... :/
Kasumi was probably the first character model, that they did for DOA5 and when they gradually improved with the later character models, they never went back to change Kasumi, because of time restraints.