I have seen some users commenting on Lemmy using a Mastodon account. I tried to login Lemmy using my Mastodon account (on the web) and it didn’t work, like account not found, maybe it tries to locate the Lemmy instance account.
I have tried to do this on Jerboa, Thunder and Liftoff too and still couldn’t work.
Pixelfed just added this option, utilizing Oauth2 I think. This might become an example on how to implement it for others like Lemmy.