Categorise in what ways, content-wise or relay-policy-wise? I don't think so, I know only sites that allows filter like paid/free, countries, software. This would also require clients that support it, currently most client try to fetch events from wherever they are.

content-wise would be great

Yes, that would be interesting, indeed. However currently I don't know about any client where you can select specifi relays before posting. This would be a prequisit for such relays. I don't think that such a UI would be understood by most users.