While on paper this looks good, this would be bad in practice.

There are too many variables in deciding who is playing well vs who is not.

Is the tank who single pulls because his healer is new and it's a low level dungeon and his DPS are both lancers with no AoE's playing poorly because he's only single pulling?

Is the healer who doesn't have to heal much in a run because players were playing exceptionally well punished because the game didn't have enough data to judge them?

Is the healer that worked their butt off to keep people alive because there were enough new players in the run punished because the system judges based on DPS done in deciding who gets the medals?

Is the healer that doesn't DPS at all not penalized because you can't consider DPS in the judgement because of the above situation where a healer can't DPS due to actually needing to heal?

How do you judge the cause for someone's output in game? Especially when the actions of other players can impact the game?

Do you punish the Dragoon or the Monk because the tank kept the boss' rear pointed in a spot where they couldn't do their positionals? Are they supposed to sit in enemy AoE's to ensure their DPS stays high enough?