Is the Lemmy logo trademarked?

By a strict legal definition, no, it is not trademarked, it is instead copyrighted. The CC License it uses is a special copyright called a copyleft which allows anybody to use it for free. (In this case with a restriction that you give attribution.)

Google cc by sa. It means it can be used with attribution and as long as your work also has same license.

However, not sure if that means it can't be trademarked.