How do I get an S in mid lane?

well an S or higher is considered a perfect game.

so you need to play perfectly.

it’s champion based so you just need to perform better than the average players with your champ in your rank.

People must seem to feed their asses off hard with neeko because I’m getting S+ when I’ve died like 5 times

CS matters a lot for grading. Looking at your match history, your cs is generally pretty low. It looks like you average somewhere around 3-4 per minute. You want twice that, regardless of grading.