ah, I've just remembered that they use this word in the TV show 'Shadowhunters' to refer to regular people who don't know about the magical world. It's short for 'mundanes', and is comparable to 'muggles' in Harry Potter. It's a fictional term.
If this has nothing to do with the TV show, then the writer is being quite sarcastic or tongue-in-cheek here. In this sense, it could be considered as a slang word for ordinary, everyday people similar to 'normies'. Again, it would be short for 'mundane'.