Could the fuel filter be the problem if your car has a rough idle when stopped at a traffic light?

Yes it could. Why not replace it? It's cheap and most people never do until they stall some dark lonely night.

It depends on what kind of car and what engine you have. Go ahead and replace the fuel filter although I don't think it will cure the rough idle. If you have a carburetor, it may need cleaning or adjusting. If you have fuel injection, try some injector cleaner in the tank.