Why do you think a subjective feedback mechanism is a better idea than the existing Blizzard Armory or Raider IO score (which uses Armory data) which is objective (less subject to human bias)?
Have you seen the toxicity in PUGs? The idea that you think positive feedback would not be overwhelmed by negative feedback is naive. The forums here had up/down votes removed in favor of a simple flagging mechanism because down votes were so abused. Now people abuse the flagging system, using it as if it were a downvote, or to censor posts they don’t like (it’s only supposed to be used for a handful of specific TOS violations).
The likelihood of a negative review is high for singular mistakes that didn’t even wipe the group or cause the key to be down-graded (missed timer). Positive reviews will exist but they will be drowned in a sea of negatives. That’s just the culture of PUGs in current WoW Retail.