The issue is more likely that lemmy won’t be able to see the post from mastodon.
Lemmy will only be able to see mastodon posts that were made to a community or in reply to a lemmy post. Essentially, the mastodon content needs to have connected with a community. Otherwise, as lemmy doesn’t have the idea of simply following users and everything they post, it isn’t pulling in any post from any person.
If you want to reply to mastodon content from a community based platform, you need to use either kbin or friendica (though I’m not sure federation between friendica and lemmy/kbin is working well right now).
Thing is, as I understand, on kbin, you don’t really get a feed of the people you follow like on mastodon. So it’s somewhere between lemmy and mastodon.
So you have option, but none of them are great right now and it depends on works for you. The fediverse still has growing to do yo really make the most of this interconnectivity.
For me, having a mastodon and Lemmy account seems to work the best, as their still pretty different spaces.