So maybe a plugin like User Notes? Where each member can apply their own private note? Or something viewable by all?
A problem with having things say public if the “labelled” user is profiled by an outside source. They might take offence. Unless it is something they chose.
There is an unmaintained Discourse plugin called User Feedback where users can rate and if configured leave a review. However same issue it could cause calamities if users get offended by what is posted.
I had shall I say an immature user flip out over a 
reaction.
A fellow long ago before Discourse added mute user option created a tamper monkey script that allowed the user to add ppl to hide post and for fun had labeling. It decorated admin, Mods, topic Op and you could label users. Each user could have essentially a browser theme component. I could try and see if I can find the script as imagine wouldn’t be too hard to convert to a theme component
Maybe using custom user Field to add user and label.