Currently there’s no user setting on Mirror to adjust the royalties. However, you can visit the contract on Optimistic Etherscan after the first collection and adjust the setRoyaltyInfo value on the contract.
You will need to input two settings. First, the Ethereum address of the recipient—make sure it’s an address compatible with Optimism, like an Ethereum EOA address. Then, add the royalty setting in basis points. In other words, input 1000 for 10%, 500 for 5%, or 100 for 1%.
Lastly, depending on which secondary marketplace is being used to trade your NFTs, you may need to configure royalties in their app as well. is the most popular NFT marketplace on Optimism and they require users to set royalties via the collection settings page if you're the owner of the NFT collection. You can read more about Quixotic royalty settings here.