Yes you can earn decent amount from airdrop alone. The thing is you have to be able to join plenty of them, and most airdrop as far as I know give tokens that almost have no value and if it has you can't sell it because it cover the gas fee. I would rather spend my time doing bounty rather that spending my time doing an airdrop. My first earnings came from airdropping though. But you can do both if you want to.