Viewing a Lemmy account’s posts from Mastodon seems to work. Going the other way, viewing a Mastodon account’s toots from Lemmy, does not work.
Pixelfed can see Lemmy and Mastodon accounts, but it does not show the posts or comments from those accounts. The reverse is also true: Lemmy and Mastodon can see Pixelfed accounts but not their content.
At least these were my results the last time I experimented with cross-platform interaction. Maybe there are some tricks I have not learned yet.