ARAM grades compare you to other performances with that specific champion on ARAM (the algorithms are different than they are on SR). So you're being graded against other people subject to the same limitations that you mention (no wards to kill, reduced CS and increased tendency for it to be split among the team, etc).
The issue with the grading on ARAM is that it rewards playing more for score than what the team needs. It's going to be hard to get an S on Malphite or Alistar if you go pure tank, because the top 5th percentile of performance is going to be on the people who went glass cannon AP and were able to nuke down squishy targets and get a strong KDA, deal more damage, and get a fair share of CS. Unfortunately, playing this way doesn't always yield a win (ability for that Malphite to play glass cannon depends a lot on the team comps at play).
So you sorta have to decide if you want to play for grade or play for the win. Personally, I'd rather play what the team needs to have the best shot at winning, even if it means I get a low grade. I've done tank Lissandra a few times, and was able to be provide the engage and lockdown that the team needed, even though I wasn't dealing a ton of damage and often sacrificing myself. Got like a C- on that game, but I also had teammates thanking me for building what the team needed.