People undervalue A's. Everyone wants an S, however A+ means you did better than most Miss Fortunes.
Key points: CS is terrible, 214 at 42 minutes is piss poor for an ADC. I don't mean to be offensive with my language, I'm just trying to stress the importance. Of all of the lanes, ADC tends to have the very highest by a good margin. Yours did not reflect that. Your Kill participation was great, being 42/55. Typically, people that don't get an S have bad participation, but you're fine in that regard.
I'm not going to go and look up exact details, but I'm sure your ward count was low. Warding is a MAJOR part of grading and MMR. Riot stresses vision and warding hard. Ward a lot. If your trinket isn't on CD, put it on CD. Easy rule of thumb. You don't need the perfect wards. I don't know if you've ever seen Sky Williams on Youtube, however as silly as he is, he had one video on tips or something and it involved warding. Always ward. It doesn't matter if you put 3 wards in Baron Pit, at least you have vision of Baron. Always better than no wards.
Damage done and taken is also a large factor. However, 26/16 has me feeling like damage done isn't an issue, and 0 deaths means not much damage taken. Good job.
Objectives. How participant were you in objectives? Did you help take towers? Unless you got a shot off on a tower, the game doesn't register as an assist. So getting a shot off on the dragon, towers, I think even like blue and red buff. Baron/Herald too.
Gold earned is huge, but falls in line with the other points so that doesn't need much discussion.
Overall, I'd say CS killed you HARD. 214 at 42 mins for ADCs, MF namely, is just terrible. This means you probably weren't farming much, so probably not taking many towers if you weren't taking minions. If your CS had been something like 300-375 with only like 10 kills and maybe a few deaths, you would've gotten an S. Also, wards. Wards are easy though to fix, CS is harder.
Hopefully this answers your concerns.
Oh, and P.S. MF is pretty meta, score is relative. It's much harder to score S on meta champions. Play something like Ashe, Twitch, or Urgot and you'll have a much easier time.