YggdrasillLike mohawks.
LOL, I just tried to imagine *that*...

But you're right, Lisa's and Maggie's haircuts cannot cast such simple and smooth shadows. If their heads are covered with triangles, we should have seen number of small shadows instead. Maybe they tried, and that looked too unusual (remember your own picture,
The Mask), so animators have decided to go with the simpler version? In the Movie, I never noticed that their hair looked wrong.