they are entirely different contexts of communication. mastodon is a 'microblog' platform, where one person tends to broadcast to a group of subscribed users
lemmy is just forums with threaded discussion.. many to many as it were.
personally, ive always generally hated the twittersphere (microblogging), and avoid it mostly. but it is nice being able to subscribe to the small number of microbloggers i do enjoy thanks to mbin's built-in integration.