This only works if the mainstream audience will indeed be attracted. Those who dislike Justin Biber, or just aren't interested, won't join a Justin Biber fan group and those who don't like Bitcoin, or aren't interested in it, won't join a Bitcoin fan group. As long as Nostr appears to be a bunch of bitcoiners talking to each other about how great Bitcoin is, it will be joined by bitcoiners who wish to talk to, and hear from, other bitcoiners about how great Bitcoin is.