Can you explain this to me more. It seems mastodon account can be used to post on Lemmy, but is the opposite possible? Can I use Lemmy account on mastodon?
Lemmy does not normally display microblog content from Mastodon. You cannot follow users and see their posts listed on their profile or elsewhere, and as a result you cannot see or comment on their posts either.
Mastodon users can, however, tag a community on Lemmy the same way they can tag a user. If they do so, their post will appear as a thread in the tagged Lemmy community, and Lemmy users will be able to respond to them.
Mastodon users can basically pop up in the comment section any time, should they stumble over it for some reason or another. This could happen in many different ways: They could follow a hashtag, someone could share (boost) content into their feed, they could be mentioned, or they could read the Lemmy community and manually open the thread in Mastodon to reply.
I think its partly because the Federation process takes time (which can throw off any more precise notifications between instances) but much more significantly because many of the most active Lemmy users really hate bots.
I think that's a functionality that would better be integrated into the client's. Idk if there are any out there already that can do it though. As someone who didn't use the bot on reddit I felt it was kind of annoying on reddit to always have two or more of the top comments be completely unrelated to the subject. It felt like spam