can the timing belt be put on backwards on a kia optima

Not really, you put the timing belt on the way it came new. You do not turn it inside out. On the new timing belt the teeth would be to the inside of the belt, the smooth side is to the outside of the belt.

There should be a mark on the belt indicating the direction of rotation. If it was installed backwards (not inside out) it might fail early.

I never had seen a directional mark on a timing belt, but the last Ford I did was 35 years ago. So not to say there is not one.