No, there is no difference between the mileage of an AMT or manual transmission. Though, getting the best mileage from a car is mostly dependent on the way one drives it.
AMT is a better choice in case of a stop-and-go traffic situation which we have in plenty here. In case of an AMT, you can feel the gears shifting as there is a slight jerk when the shift occurs.
Maruti Suzuki Alto K10 has always been a value for money car and with recent updates it has become even better. The 1.0-litre K-Series petrol engine is quite refined and fuel efficient and the 5-speed Auto Gear Shift is quite in sync with the engine. The same engine and transmission unit is being used in Celerio. The car performs well and provides you a hassle free driving experience in city traffic.
Maruti Alto K10 AMT drives like any other automatic transmissions car, you just have to put it in drive mode and then you can enjoy the drive, its auto gear shift also allows you to change gears manually, when you want to do so. The mileage of the car is quite good and in the city traffic it gives mileage around 18kmpl with AC and around 19.5 without AC.