Yes, Zarya can definitely carry you on competitive games down from the low ranks to the high ranks. She can function as a D.P.S. if your teammates are doing poorly and can function as a flank to kill off supports herself. I would highly recommend you watch your VODS or replays in your Overwatch matches as Zarya and try to check where you went wrong and what you would have done to mitigate the consequences of your teammates' actions, etc. The thing with Overwatch on solo queue is that you can play as perfectly as you can, but your teammates are bound to make mistakes, and it's up to you to try and do something about their actions.