Not a lawyer, but I believe if you want to take content others have posted on Reddit and repost it here, you would violate their copyright on that content. You would want to get their consent before you post it here or else the instance you post them to could start getting DMCA requests and could ultimately be sued. I’m guessing you could also be sued for writing the software that violated the copyright.

It’s not likely to happen but it could. My concern would be less about what ill effects you and/or Lemmy might incur and more around the ethics of taking content belonging to other social media users and reposting it without their permission. Reddit TOS says users retain copyright on their content, so it’s other users’ rights you’re violating in doing so.

Now if you’re talking about a bot that reposts only content you have posted on Reddit, you’re probably in the clear.