There's a lot of things that factor into the answer, but I think overall it's gonna be pretty random. Some instances are on domains without "Lemmy" in the name, some don't include "Lemmy" in the site name configuration, and in the case of some like my own instance, I set the X-Robots-Tag response header such that search engines that properly honor the header won't crawl or index content on my instance. I've actually taken things a step further with mine and put all public paths except for the API endpoints behind authentication (so that Lemmy clients and federation still work with it), so you can't browse my instance content without going through a proper client for extra privacy. But that goes off-topic.
Reddit was centralized so could be optimized for SEO. Lemmy instances are individually run with different configuration at the infrastructure level and the application configuration level, which if most people leave things fairly vanilla, should result in pretty good discovery of Lemmy content across most of these kinds of instances, but I would think most people technical enough to host their own instances would have deviated from defaults and (hopefully) implemented some hardening, which would likely mess with SEO.
So yeah, expect it to be pretty random, but not necessarily unworkable.