Can I use the output of mobile charger as a DC source for my circuit?
It depends.

If the mobile charger is just a DC power supply (true in most cases I believe), then yes. You will be able to draw 5V at whatever current it can provide although it might have a minimum current in order to regulate correctly.

If the mobile charger contains the actual charging circuitry (as opposed to the phone) then no, because it will attempt to charge your circuit and apply various amounts of voltages, constant current, etc.