In addition to what the other commenter said, I'll say "technically you can do whatever you want."
If you want to create a channel to talk about cats with local people, you could make the channel and share the QR/link and some sort of "this is for meshtastic, look it up" sign at local cat related places like pet stores or shelters or whatever, but who knows how many people will actually join it.
You could also try (and I haven't so if you do try let me know how it goes!) to create a new topic on the MQTT server (msh/us/cats or something like that) and share the link with meshtastic groups or cat groups online. I'd like to know if it's possible to create a new topic on the server by just adding one in your settings, so definitely let me know! Note, only people with the same topic in their MQTT settings will be able to see your posts there, it's like a room within a room.
My local area has a set up like the aforementioned dayton mesh, and another dude nearby who created a channel as well but I've never made a contact on it, so people are indeed doing it, but at least in my area it isn't really feasible to be that specific on topics yet. Mostly it seems to be LongFast, Localmesh (like daytonmesh or ohiomesh type stuff), and then private channels for friends who all play paintball or camp or something like that. There are probably many that are sort of ephemeral, like they'll spring up for a protest or concert or hamfest, and then they'll all just be deleted at the end of the event.