>>47690354easy, this game calculates all sort of damage input/output, speed boosts, stun, etc, all the time. You could create a metric based around how long the opposing team was stunned for by you. Of course this is one of many metrics. You could also have this mean more for certain situations (certain zones, certain opponent characters, last two minutes of match, on opponents that have popped an alt/have multi kills, etc.)
These metrics could also vary per rank, and could become more nuanced/focused on something else
Is it perfect? no. But its better than "did you happen to land on a team of people who decided to try, know what to do, didn't fuck around, and also win?"