I'm not really sure... but the way I see it it's probably fair game.
Communities aren't something that somebody (reddit, specific moderators, etc.) owns, they are just concepts that people latch onto. And, for me at least, I would rather see popular communities exist here if people want them to, especially since you can have multiple communities under the exact same name on different servers in the fediverse.
In other words, if you want to bring over a specific reddit community I think you should just do it.