Where did the use of 'mundane' for the rest of society, come from. Is it an offensive term? Will someone please explain it to me. Thanks.
It was current in the Society for Creative Anachronism in the 1970s, which is the earliest subculture usage of it I know of. They used the word to refer to people and things outside the society with an admixture of pity, contempt, and superiority.
So yes, within subcultures it is used as an offensive term, if only a mild one.
Even outside of those subcultures it has overtones of the bland and boring. It is not a neutral term.