I'm gonna drop some facts on you.
First of all, it's their algorithm and there is math behind it. What YOU think you deserve is completely irrelevant. You're not important to them in that regard.
Second, it takes everything into account. Kills, deaths, assists, CS, wards, towers, objectives, damage, healing, etc. It also compares you against other people playing the same champion. So, compared to other players, you earned a C+. You can believe what you want, but lots of people simply did holistically worse than you which got you your grade.
After that I called you a name, but I won't post that.