I'm following the furry community on yiffit.net using my mastodon account, and I saw a post show up on my mastodon feed. So I opened up the thread in another tab, and I've been faving/unfaving the mastodon post... and the score on Lemmy goes up/down immediately. So yeah, it looks like faves from mastodon are translated into upvotes on Lemmy.
I've also seen likes on my PixelFed notifications come in from Kbin.social users on this post that I sent to Lemmy from there - I'm assuming those were upvotes on their end.

I'm now thinking that some of the upvotes on that thread are actually likes and shares from mastodon. Edit: this might only be the case for new posts.

@bristle Pro tip – when replying to a Lemmy thread from Mastodon, remember to tag the community (i.e. @tech) in the reply in order to ensure your reply actually shows up on Lemmy. If you don't, it won't always federate back to Lemmy.

It seems to federate without tagging when the thread OP is a native user on the Lemmy instance, and when directly replying to a Lemmy user. Still unsure exactly when it will and won't.

@bristle If you follow @tech, that's basically how you subscribe. It will boost all posts and comments made within the community, so you can see them.

I am not a fan of it boosting every comment from every post, but that's actually required in order for those comments to federate to Mastodon (otherwise, you just wouldn't see any comments from the Mastodon side).

Subscribe by following the community handle, like @tech. As for voting, some of us don't mind not being able to do that. If I decide to start caring about that, I guess I'll get an account on a proper server that handles this feature.

@bristle @tech I assumed that a masto fav became an upvote, and that Lemmy would lack downvotes. The latter is apparently wrong but there's at least a chance favs work like that.

@bristle @tech It's really cool that it's possible :3 but I kinda wish there were a way to mute Lemmy communities from a Mastodon home timeline without muting the entire community (unless there is and I haven't found it in poking around).

It is very nice being able to be on masto and have a list that just streams in any subscribed posts and comments though ^^

Edit: Also, subscribing to a community by way of Mastodon involves following the community tag. The downside to this is, as I mentioned, it appears in your Home timeline

@bristle @tech I think you nailed it, "the interfaces don't exist yet", we are presently(?) at the point where we can send email, I mean activity pub mentions, but not at a point where say Mastidon and client know how to cast an up vote back to a lemmy. just that, if we built those interface and interoperabilities, they would work in neutral way... like email!