Not entirely I believe, players get higher grades when playing against bad defense. Grades can be an indication of quality, but you need to take into account that there are several factors at play with match rating. First is game shape; check wether the player has received good minutes. Also offensive tactic can influence grades. If you play LI with a guard with bad driving and IS he will get a lower grade I believe. I usually just look at the skills, that says the most. Then dont forget to check if the player is foul prone; I try to avoid players that get more than a foul per 10 minutes.
If you have real dead weight, I'd cut it. I keep 12 players (incl. Trainees) at most plus some 1k salary scrubs in case I need them in scrimmages.