I think I would limit that to forks of one’s own notebooks at best. I’ve had a few cases where I accidentally liked a completely unchanged fork before realizing it was not original work. Now, at the moment I’m sure it’s mosely someone (presumably) using a fork to save a notebook for later, but it still worries me.
I kind of hope that Observable will manage to avoid the issues that seem to be inherent to likes/retweets/upvotes in social media sites (especially the part where popularity contests drive the platform to be less and less about producing original work). If I people could just copy other people’s notebook and present it as their own with a click or two I think that would make things worse in the long run.