Yes, you can. After their last release you can follow Lemmy communities with Mastodon and add comments to posts. Just enter the URL of a Lemmy community in your search bar to find the account profile. And in case that doesn't work you might try search for @[CommunityName]@[lemmy.instance] (there are some issues to be wrinkled out still).
Since I already have an account on a mastodon instance I first thought I can just connect to lemmy instances using that but that is apparently not possible.