Yes. You can be a hardcore raider, hardcore m+er, hardcore pet battlers, hardcore collector, hardcore pvper etc.
The problem with understanding this comes from elitism found in some circles (notably people from the original main activities of the game - raids and pvp) who think that you have to partake in their favorite activities in order to be classified as hardcore.
I only do heroic raids and do like 1-2 mythic bosses per tier tops. At the same time, I consider myself much more hardcore than like 90% of mythic raiders due to my collections being much much better than most of theirs. This doesn't have much to do with the topic itself, I just like to include to piss those people off