Having a license won't help you combat piracy. People who steal games and assets, know that they're doing it, and don't care.
The only issue where you'd need a license is to let people know if they're allowed to use your assets commercially - i.e. put it into their own games. You could write it in the description, but that's risky for the users. If they have a "readme" file they download with the asset, users are better protected in case there's a dispute.
If you report a game to Itch.io / Google, it's up to you to show it's your game and a license alone won't help you. You'd need screenshots perhaps of the game in your software, and of the publishing date that shows you published first.
(+ do what redonihunter suggested by putting in links to your itch.io pages or website etc. in the game itself)