Op.gg and similar are merely estimates of your mmr, not your actual mmr, which is hidden and only riot knows.
Your grade and performance has no impact on your mmr, only if you win or lose (role does not matter). How much your mmr changes depends on the mmr of those in the game, so if you're in a premade with low mmr people, you'll be facing people that the game 'expects' you to be able to beat and so your mmr gains for winning will be relatively small (but your potentially mmr losses will be larger). Conversely, if you are in a premade with people who have a higher mmr than you, you'll end up in a game against higher mmr people that the game expects you to lose against so you'll gain more mmr for winning (and lose less for losing).