You could probably run a server on an rPi but I don't know if it would be quick enough. No need for a VPN.

You don't need to run you own server to use Mastodon, of course. I thought about joining https://mastodon.me.uk earlier, when it was accepting new accounts, but couldn't think of a use for it.

You can see what sort of people are using it on that or any server without having an account.

You can search for tags, like #folkmusic. I didn't find much.