Oh no not at all. I wish it were that simple lol!
I went 7/0 KD with a bunch of assists, got 2 drags, helped take baron, and I was warding like mad with
while peeling a ton for my adc and in team fights.
The more wards you uncover with the lense and destroy, it helps as well.
Warding in crucial spots on your lane and in the jungle tally up in the grade as well...plus, it factors in how well you play Support among other people that play Lee as a support...if they ever do. Most use him for a bruiser or sub-tank...so getting an S on him could require you to get like 12+ kills and 0 deaths which will be tricky unless you ks your team mates in team fights.
I've only played lee a few times, and not one of my favorite champs to be honest.
But warding, creeping, taking turrets, getting kills and taking minimal deaths while racking up some KP/A helps tremendously.
Time in the match is also a factor...if you spend a lot of time returning to base, that does shave your grade down...as I went like 9/0/14 with 70ish minions kills, won my lane with the adc (I took the towers down) but I was consistently healing by returning to base and got an A- within a 20 minute match. I was told I needed like 100 minion kills within that time frame, but in the match I stated previously, I had less than 100 minion/monster kills so...
It's really hard to say how easy it is to get an S on lee, but this was with Heim and Quinn...
[zombie-brand-clap]
So Ward like your life depends on it (and it does lol)
Be around for the majority of the killing/fights
Drop turrets
Solo dragons if you can get away with it
Wreck minions in your lane and roam to help control
Avoid getting killed as much as possible, and watch out for baiters
Pretty much, general gameplay.
Oh, and when you sweep/ward...avoid popping too many wards at once. The limit of wards you have out versus the 4 you get with the Eye really hurts your score (as it seems on my end).