Apparently going 18/1/17 as Jhin on Aram doesn't give an S

you lack CS

CS? wat? lol, its a mystery to me also how these are decided.

Tell me about it. Played Sivir in aram, went 7/5/41 with 173 cs and got an A+

It's cause it's a bad score. You need at least 120 kills, you can't have 1 death, and also 200 assists min. plus 800 cs. Also the game must be ended before 12 minutes. Good luck.

Kill participation?
Damage % for team?

For ARAM, you need near 100% Kill Participation for S's on most non-frontline champions. Also, 35~40% of total team damage.

My guess is you played TOO defensive, ended the game with a ton of extra gold (or were on the field with your starting item for over half the game), your team members did a large amount of the damage and you got a lot of last hits.

meaning you STOLE the kills so NO S for u