It depends. Can if be earned income? Sure. Then you are not only subject to federal and state income tax, but also self employment tax (social security and medicare).
There are other ways to get earned income, like consulting fees.
There are also ways to get more into a ROTH than the yearly contribution limits.
With a backdoor Roth, you basically start the money off in a traditional IRA, transfer it to a Roth IRA and then pay the taxes you owe on that money now so that you can let your investments grow tax-free and enjoy tax-free withdrawals later. It's that simple and it's perfectly legal.