Anet already tracks this number for internal purposes. Most people would like to know how they stack up as X class. Anet can easily make that number visible to the player themselves and not use that number for matchmaking purposes.
This would be a simple quality of life change that allows players to know which class they play best, worst, and close to their average MMR.
Northern Shiverpeaks (NSP)
Thief (Daredevil)
Commandant of
Pistol-Dagger and Apex Predator