I don't see how a mirror could work. Would your account info/password be copied over to all instances in case you switch? Anyone can spin a Lemmy instance; anyone could copy your account that way without your knowledge. It is good that it lives only on one instance. Your old comments and posts will still exist in other instances where they were copied over but you will have to continue with a new account. If you want to collect karma it can be annoying but otherwise it will not make much of a difference.