I would love it if there was an ingame grading system and it needs to be very forgiving in dungeons. What it should punish is the most basic things like never using aoe on groups (after you get an aoe ability obviously), never using any form of mitigation, constant overhealing 0 dps healers, constantly breaking combos and ignoring your buffs all the time. And it also needs to take into account how new a player is. Not realizing you even have mitigation when you tank sastasha for the first time is something entirely different than never using mitigation as lvl 80 tank.

I doubt however this dream will ever come true however. It would be tons of effort to get such a system right and it would create a lot of fuss with people afraid of it, I dont think the devs are interested in either at all.