There are some niche communities on fediverse, speaking as primarily a Mastodon.art user. The hashtag system exists as a way of making a toot available for search, as most fediverse platforms don't have full text search. Hashtags essentially work like twitter hashtags but better.

Fediverse is better suited to niche communities, anyways. Feel free to make your own instance, but remember that running an instance is a huge responsibility: You have to suspend the bad actors, ban people who post hate speech, and generally ensure that your instance is running well. Admin work is psychologically stressful; just ask any of the reddit mods who quit over the API changes making their job impossible.