Pixelfed is more similar to instagram and Mastodon to X (Twitter). They are different ways to access the Fediverse, Lemmy and Kbin are similar in that way (though Kbin can also use a interface similar to X).

And posts in Mastodon and Pixelfed can be seen by each other's users as long as the respective servers are federated. So choose your preferred app/interface and post freely. From what I've seen Pixelfed is used mostly by photographers.