Lemmy is search indexable. It includes the necessary metadata headers to link back to original domains as well, something other ActivityPub implementations forgot.
The user base on Lemmy is tiny compared to Reddit, though. Same with Mastodon vs Twitter. The problem isn’t search indexing, it’s the user base being several orders of magnitude smaller.
If you search for topics where Lemmy has a lot of posts (Linux distros, for instance), you can already find Lemmy in the search results every now and then.