Should I answer Did you participate in Defi or a DAO as YES in my tax return? I traded cryptocurrency in CoinBase and already reported capital loss/gain on converting.

Possibly, don't confuse the platform with actions taken within the platform. These organizations were created by developers to automate decisions and facilitate cryptocurrency transactions.

Purchasing tokens is not a taxable event, so you could say "NO" if you only purchased tokens; however, if you sold, traded, bartered, exchanged or disposed of currencies, you would say "YES".