You can actually write posts from a Mastodon account that look well on Lemmy. All you have to do is follow a few basic rules.

1. Your post must include a line break. Everything above the break will be converted into the title of the post, everything below into the body.
2. Don’t put any hyperlinks into your title. That includes hashtags and @ mentions. See the original post for why
3. Keep your headline short. Lemmy has a character limit for the headline and if you exceed it your title might just end mid sentence
4. Put the @ mention of the community at the end of your post. It’s nothing but a technical necessity, so treat it as such and hide it away at the bottom of your post

If you follow these rules you can write posts from Mastodon