Currently a users has to trust repo owners if their packages are stable or not and repo owners have to write a warning into the project description. So it would be more usefull if there would be a flag for unstable/experimental repos the project owner can set in the backend. If this flag is set there could be a warning sign displayed in the frontend beside the project name. In this way you can see at first sight if a project is stable or not.
And who will be setting this flag? The owner, who does not bother to put this statement into Instruction field already?
Copr admins? We neither have time nor knowledge for this.
I would like to point out that Env&Stack Working Group have idea of Playground repository
Where some set of copr repository will be marked as part of Fedora Playground and will have to have some quality.
Copr already have all bits in place (server side, admin interface, dnf plugin). Now it is on Env&Stack Group to formalize the process and properly introduce it to Fedora.
I doubt that Copr itself can do something more.