Lemmy search works pretty well on larger servers, and they're indexed by major web search engines.
The microblog side of things is worse, with Mastodon long having near-useless search because it might "encourage negative social dynamics" or some such. Some other software, such as Akkoma has had better search, and Mastodon has recently improved somewhat for accounts that opt into being searchable. Mastodon directs search engines not to index most pages.
Some people get very upset about attempts to build general-purpose fediverse search tools.