to put it another way
If I happened to go outside and meet a busty old hag there's nothing stopping me, in my mind, from perceiving her as Yukarin if I wanted to. I could totally just attribute all my wildest fantasies onto this unsuspecting woman, and I guess if I did it long enough the illusion would get stronger. So in that sense you can "control" reality through perception -- by selectively choosing how to interpret what you perceive, and putting your own spin on it.
But I cannot "control" reality to such an extent that Yukari - in full 2D form - will walk into my room right now and sit on my bed ;_;. That would go past perception and into mental illness, I'm afraid.