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.
No.
Accounts are tied to the instance they were created on. Accounts are not federated, only content.