This is only true if the mastodon instance in question has gotten copies of the post—if nobody on an instance follows your pixelfed account, and nobody on an instance follows an account that boosts posts from your pixelfed account, then (with a few exceptions) the post won’t appear in that instance’s Federated feed or that instance’s hashtag feeds
Have you considered posting them on Mastadon too? You might get some interaction with such a greater userbase.
If you’re using relevant hashtags, then anyone browsing/following those hashtags would see the PixelFed posts on Mastodon anyway.
But you could create a Mastodon account and boost the Pixelfed posts for additional exposure as well.
This is only true if the mastodon instance in question has gotten copies of the post—if nobody on an instance follows your pixelfed account, and nobody on an instance follows an account that boosts posts from your pixelfed account, then (with a few exceptions) the post won’t appear in that instance’s Federated feed or that instance’s hashtag feeds