Looking at the Mastodon and Lemmy documentations, yes it should work.

• Lemmy accepts posts with the: Page, Article, Note, Video and Event activities.
• Mastodon accepts toots with the Page, Article, Note, Video, Event, Image, Audio and Question activities.

As you can see there’s a large overlap between the two, so I say it’s likely that it will work. I could bring this even further by having a look at the plugin’s code but unfortunately I’m alergic to both PHP and SVN abd wordpress uses both.