Average bland fights I give a 5, and here is pretty much how my rating scale goes:
Added points for: Good combos over 40%, good reaction punishes, good sportsmanship, using a character other than Scorpion or Ermac, etc.
Deducted points for: Constantly spamming special moves, no combos, using Scorpion or Ermac, whining on the mic, playing like a douche, abusing broken elements of the game, glitches, etc.
I'd say most of the time I rate people about a 6-7. Also to the people who always rate 10's, you're not doing the good that you think you are. Ratings would help encourage people to get better so they get rated higher. If a noob wins and gets all 10's they won't feel a need to progress their game at all.