If you want the post to circulate better on the microblogging platforms like Mastodon, then including hashtags in the post title is useful (post body does not federate), as that is an important part of their discoverability features. Personally, I think that Lemmy should add a dedicated text field to the post submission form for hashtags to further improve interop (and to avoid cluttering post titles). Lemmy doesn’t use them itself and most of Lemmy’s users came from Reddit where they’re also not used, but on the Fediverse I think it would make sense as a cultural norm to use them.

I lost my best friend to Hastags in 'Nam. Keep em off my lawn!!!