If you want to go, put a mirror on the headrest. The lights reflect in the mirror and she can watch the lights on the mirror while you go through.
We went through a drive thru lights display that the city sets up in a park every year. Most of the lights were angled so that you had to be looking forward at them to see them, but ds apparently saw more then we realized because he was calm through the lights display and as soon as we left the park he started fussing. He's 14mos old, certainly not old enough to enjoy the pictures and recognize what the lights are, but he definitely got some enjoyment from it.
My 3yr old dd did just fine as well although she's ff'ing, and she's in a Radian so lower profile... we told her ahead of time which side of the truck to look out so that as we came up on each new display she was looking the right direction.
I say if you guys want to go, go for it. She might enjoy it a little, and you guys will enjoy it. And no, I wouldn't take her out of her car seat, but I'm sure she'll be able to see the lights even being rf'ing.