IME Lemmy is more predominantly far left although a very small userbase.
Mastodon has islands of different culture on both the left and the right that don't interact much save a few of the largest instances. Some of the medium-sized ones vouched by joinmastodon have a lot of far left people. There's also a lot of mastodon forks which have been rebranded and closed off (at least one is far right), which complicates things. If you look at Mastodon blocklists you can get a feel for the boundaries.
Mastodon has islands of different culture on both the left and the right that don't interact much save a few of the largest instances. Some of the medium-sized ones vouched by joinmastodon have a lot of far left people. There's also a lot of mastodon forks which have been rebranded and closed off (at least one is far right), which complicates things. If you look at Mastodon blocklists you can get a feel for the boundaries.