There are rss bots that post stuff on lemmy, like for hackernews. And then as another user mentioned there are some servers like 50501 that mirror reddit and cross post to lemmy.

I think the general problem with such things is… they post a lot of shit. Sure the top content within a community -might- be interesting but you’re also going to get pointless junk that just fills up the fediverse. Not to mention the other issues with lack of interaction. I think the unfortunate reality is that the long term best thing for the community is hand curating content and posting it yourself. I say unfortunate because that’s more work than a bot, but you’ll be better able to grow a community. Plus some people like me will just block any bot i see because they generally are a waste of my time