Is it appropriate to email a professor about something unrelated to class?
I assume you mean something related to academics rather than a completely random meme/cat video.

If so: yes, it’s totally appropriate. Discussing cool research topics with students, especially when it’s free from the context of grading, is one of the most fun aspects of a professor’s work. I absolutely love it when students email me links like that.

And if the professor thinks it’s a waste of time, they’ll reply with a curt thanks and ignore the link, or simply ignore your email altogether.