Barring changes (which are probably coming) in the software, going from Lemmy/Kbin to Mastodon is going to be problematic (unlike the reverse). Mastodon people will only see your hashtags if their instance federates with the applicable Lemmy/Kbin instance, which it probably only does if members of that server have tried to follow a Lemmy user or thread - which is unlikely.

So by adding hashtags you’re mostly just adding extra words that Lemmy users won’t have any use for. You may get some traction on very large Mastodon instances if you’re also on a super large Lemmy instance, but super large instances are also antithetical to ActivityPub.