I don’t think it’s technically impossible - all the information that another site needs to properly interpret some activity is in the JSON that’s sent. I get the sense that it might be unrealistic to expect Mastodon to make the necessary changes though. It seems more of a political issue than a technical one.
Lemmy doesn’t seem to get much recognition in the wider Fediverse - it tends to get bundled as part of ‘other apps’. Mastodon is much bigger, so better integration with Lemmy probably gets deprioritised below their own issues and feature requests (e.g. I was reading today that Markdown support is often requested, but the base version still doesn’t have it)