A fair bit of Mastodon content doesn’t fit well on Lemmy. One mundane technical reason is that their posts don’t always split up well into the post title / post body that Lemmy expects. A cultural reason is that Mastodon users have a much higher tolerance for other users promoting things like their patreon than Lemmy users do. Even if the posts split well, and is content that Lemmy would like, bringing in the replies to it opens up a spam vector.

Lemmy let’s you impersonate other users. I used to do that with https://lemmy.world/c/tails@lemmon.website, but stopped because the above-mentioned reasons made it tricky to automate (and because I got bored with it)