Airdrops aren't always waste of time, it depends on how good you are with them, the easiest way if detecting a good Airdrops is to watch the rating on top Airdrops platforms like airdropalert or airdropbob, if it's rated 5 starts it's probably because it's listed on coinmarketcap and it's trading
The presence of a coin in the coinmarketcap rating does not mean that it is good. Here it is important that it is in the TOP 100, and not in last place. But if you have certain skills for working with airdrops, you should continue to participate in them.
Even being in the top 100 of coins ranked by market cap or whatever metric you choose on Coinmarketcap is not a guarantee that a project is 'good', however you consider that. There's no one right way to determine if an airdrop's worth your time, but just looking at whether or not something's trading should not be the only criteria you use.