How Does Support Grading Work?

Your farm and wards placed, perhaps.

I know, it's stupid, but you still have to last hit creeps even as support.

And I think Nami needs more than just 60% kill participation; I could be wrong.

It's the same as every other champion. The stats you have are based against players who played the same champion.

Zz'rot portals help with cs and taking towers, which I think lands me more S ranks. Did you know last hitting a tower is a statistic in every game? "Towers Destroyed" -- I think this sometimes helps with them, but I might just be crazy.

https://champion.gg/champion/Nami Nami gets a lot of assists naturally because of her kit, so 1/0/11 or 66% kill participation isn't necessarily impressive. It's almost enough as indicated by your A+. So you are on the right track with what you are doing as Nami.