Since most Lemmy instances won't come loaded with Google analytics, solely relying on Google crawlers I would imagine will be very ineffienct at gathering all the posts and comments.
To comment on your last question. In theory, yes it can, but practically it will be as tricky as Google's method. It will be much more user-friendly for end-user though, you just have to ask "Is the housing market about to burst? fediverse top posts" and I would imagine the results will be pretty relevant.