I actually message people with positive feedback if they play good. I often write something like u r awesome. or good job, they really appreciate it and enjoy it.
It really makes the games a lot more fun for you and everybody else whether they end up on your team the next round or not.
Yeah, I do that too. If I've had a fun round, even when I lose I will send a note and leave positive feedback.
Is there a system where you can leave positive comments for people who behave like a regular human being? Maybe give them some sort of achievement for being a decent person?
Yeah, you can "Prefer this player" on the same page where you can denote you'd rather "Avoid this player." Oddly, the former doesn't require a reason, while the latter does -- and it's entirely separate from "File a complaint."

The only negative feedback I have states I quit a session early, when I never do. I guess the butthurt jerks I've just owned so badly they never want to play against me again are unable to file "unsportsmanlike behavior" because it would be overwhelmingly ironic.