Is Sona's damage a little too much?

I might be completely wrong about this. I am asking not stating.

She isn't supposed to be an APC right? I just played a match where she was played support and went for an APC build and out damaged everyone in the game. She was one shotting people. No one was going for MR really because her team didn't have much AP.. Or so we thought.

Either her base damage is too high, or I am confused as to how a support gathers enough gold to buy enough AP items by 20 mins to one shot people. My bot lane was horrible of course and she got very fed. I am not sure if its that, or her damage is really just a little too strong.