any way to follow hashtags on Mastodon?

The friends plugin has support for keyword matchers which could be notified of a hashtag, too. But this only works for the people you follow.

But likely you’ll want to see posts of people you don’t follow. There are so called relays (see https://relaylist.com/) that could be used to achieve this. I’d be happy to add this to the plugin if someone contributes the code. So until this happens the answer is unfortunately: no.