Don't know about lemmy, but on kbin you can go to /d/lemmy.world to see what appears to be something like a frontpage of lemmy.world.
It's far from perfect though:
It only shows that instance's local communities. In other words, you wouldn't see that a lemmy.world community is trending on feddit.uk without going to feddit.uk itself.
If the lemmy server has the concept of a downvote that would reduce a post's rank, that isn't information transmitted to other instances.
I might be mistaken here, but if I understood correctly, it only shows posts from communities that at least one user on your kbin instance is subscribed to.
It will most likely be a bit out of date
If the list of (de)federated instances isn't the same for your instance and the target instance, the upvotes of entire instances might not be counted for one of them.
In future, some instances might determine ranking solely based on local votes.