Even the best attempt to incorporate all these different types of content into Mastodon is going to further complicate the platform and make more people dismiss Mastodon as too complicated of a Twitter alternative. This isn't a situation where there's no harm at best. And the potential benefit? Lemmy comments having the occasional Mastodon user?
Mastodon itself is a good enough introduction to ActivityPub without needing to make it support other things. It shows how people on different servers can share & interact with a pool of media through the same protocol. When people learn about other platforms on the fediverse, they can go check those out. Just promoting the platforms will do the job fine without complicating people's entry into the fediverse.