Why does Kitty's hair always look like a hot, ratty mess?

I think the word "fabulous" is a bit of a stretch. Just like Calista's forehead (dare I say it). Of course she doesn't look her age - anyone who's had work done would know that.

Then again, maybe that's what American guys go for, a woman with the figure of a small boy, pasty skin and bad hair. I could never understand why her character was always being hit on in this show. Sarah was a lot sexier and much more natural.

Exactly. Her hair looks better that way. If you watch carefully, it tends to look better when she's outside and the wind blows it around a bit. In the episode where they go to the ranch in Ojai (season 1), it looks so much better than usual.