This answer is not about the MMR matchmaking but more about why you have winning or losing streaks.
In my experience, you will have winning streaks and losing streaks. The only thing is that when we win, we tend to tell ourselves that it's because we are good. When we lose, we subconsciously try to blame others or to minimize our part in it (you can always do something better).
But to be honest, sometimes you will win even without doing anything, being carried. Sometimes you will have to carry hard to win. It will depend.
I feel that at the end, the most important part is the sinergy with your team mates. So many times, I happened to win or crush a "good" composition with a "bad" composition just because everything goes well with your team mates, ganks have the perfect timing, people think about the same move beforehand and are synchronised etc...
Of course your skills, the level of your opponent is related to your victory rate, but you can't do much when there is no synergy in your team. You can just try to communicate more, without flaming. Try to do the best with what you have.