No

Karma was the useless metric, not a post's score. When you upvote something, others are more likely to see it. Simple as that. As far as I know no algorithm is being trained to show you similar content, it's just a question of you wanting to give visibility to a post for any reason you might have.

Left

This reflects my experience too. After a while, I managed to find interesting people to follow who sometimes "retweet" (forgot the Mastodon name for that) other interesting people which I can in turn follow. I now have a slow feed of varying quality and I barely visit it since joining Lemmy. Mastodon looks great if you join the same server with a large group of friends, follow eachother and use it like a group chat, but, like you, I have found it really difficult to set up an interesting feed for myself.