You basically have to go there to view the NSFW content, don't you? I looked around some server federation rules and it seems like most servers block that one's local communities. (It doesn't appear to be out of ill will for the server, as most of them still allow federation of non-NSFW communities... But I imagine it's a large chunk of data to federate with very little reward since most Lemmy servers are run on little more than donations, volunteer efforts, and fumes.)