Jeeze, all these people that make these suggestions, I swear, do so in a bubble.
- Some fights are spent more efficiently healing with Aspected Helios/ Medica II/Whispering Dawn and allowing some iota of overhealing to allow for other mechanics to be covered and/or allows healers to DPS; there's really no reason to penalize overhealing, just useless healing.
- There are fights where mechanics must be done, like soaking damage or taking meteors; having a scoring based on the amount of damage taken reduces your score will incentivize people to ignore mechanics. At the same token, certain mobs, like the diresaur, have completely dodge-able attacks; unless the system could point out where you took that unnecessary damage, it's worthless.
- As it is, people will sacrifice taking down a high priority target to DoT other monsters to increase their DPS (this was especially bad in Nidhogg and have been in parties where this has caused many a wipe from too many people doing this). And people trying to avoid overkilling because parse; yelled at my own raid team for this far too many times to count.
- Where would this be applied? You say "dungeons" but that is hardly content that matters. It's something generally done for grinding; be it levels or tomestones and where gear is not really required.
This grading system does not help it the game does not then hold your hand and tell you why you fell short in an area. IE: You did poor damage; will the system tell you it's because of your rotation, or because you've been unlucky and still have that i276 weapon in Castrum Abania? That's hardly on the player if it's the latter and wholly falls on the system with it's RNG nature because for some reason, weapon drops, despite having the most impact on your damage/healing/enmity, is the hardest thing to upgrade.
Basically, unless the system says why you fall short in certain areas, this is really moot. I'd equally be concerned on how much time and resources it'd take away from the team to create and implement to actually be useful. The last thing I'd want is for some system like this to be placed in and have my main story quality go down for a dumb personal ranking system that I couldn't give two fecal matter for.