Yes and no. Mastodon instances can show Lemmy community posts in a clunky looking way, as they treat communities like a group account which boosts every post.

If someone on Mastodon is familiar with Lemmy, they can technically also follow individual people to ensure their feed looks a little less awkward. However, given the default limited character count of many Mastodon instances, it’s still going to display as the title and link to the Lemmy post at best, if I remember right.

On the other hand, Mastodon people can post to Lemmy communities and reply to comments in threads here, but it also tends to come out rough, especially if they’re unaware of the formatting differences. If you’ve ever seen a really long title that abruptly cuts off mid-sentence, there’s a good chance that was someone posting from a Mastodon account, unaware that it’s better to make a brief opening sentence as title, then an empty line followed by body text, to best fit the formatting here.

Similarly if you see someone replying to a comment starting with an @ mention to the person they’re directly replying to, chances are it may be someone from Mastodon, as replies there include @ mentions by default. Also due to federation weirdness, I’m honestly not sure if they have to keep that mention or not for it to work properly, think I’ve seen it work without, but it’s honestly easier to make an account on a Lemmy instance to participate than deal with awkward federation issues that come up between it and Mastodon.