I'm pretty sure the reason they wouldn't want to do something like that is because it would essentially just be a "Badge of Shame" for those players. It would only promote more toxicity because people would complain about having those players on their team. Or people would make fun of those players that they have a ribbon because they're toxic and they'd just start flaming each other.
Riot already said that in the future, they want to make it so you can see anyone's honor level on their profile as long as it's Level 2 or above. So when they implement that feature, you can automatically tell who's honorable and who isn't by looking at each person's profile.