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.