There's no reason why you can't Coinjoin/Lightning your UTXO's in conjunction with Monero - it's a good privacy tool and supplement to Bitcoin IMO.
If you are already using coinjoin and Lightning, then what supplementation does Monero provide?
Both Coinjoin and Lightning come with limitations when it comes to achieving completely untraceable transactions. Monero has some fancy encryption stuff going on under the hood to keep things extra confidential. Specifically, features like confidential transactions and stealth addresses make it way harder for folks to follow the money trail back to any particular person and so if if privacy is a top concern, Monero brings another layer of anonymity to the table.