From my observation of FC posts, 9 - 11 km/L is normal whether it is Vios, City, Myvi or any other cars, in traffic jams.

Idling uses fuel. Idling causes your engine to get hotter (because no airflow as the car is not moving). This causes you to use more aircond. This causes more fuel to be consumed. Idling & traffic jams causes all kinds of stuff that causes bad FC. Traffic jams are BAD for FC.