I like the idea of poniko also being a dreamer. I honestly think somebody should make a fangame where you play as poniko.
The ending is she gets turned into ubuu by Madotsuki?
What if Poniko represents Madotsuki's unwillingness to go outside and explore? While Mado goes about many different dream worlds, almost naively, Poniko is so used to it that she sees no point and keeps to herself. She is, rather, the awake state of Mado, who refuses to even acknowledge there being someone else in the room. Unless that person is a cat… And there's also the whole Uboa thing going on with her putting an effort to show Mado "the real world" or give her an eye opener. The surrounding area is pink and lush, there's birthday balloons and an island of cake - then there's a giant multi-limbed monster vomiting blood or what have you.
That's probably been discussed to death already, but given the extra angle that she dreams and explores the worlds herself, that's a different context where she is almost like the natural result of Mado becoming reclusive even in her dreams. Maybe given something like Poniko's room being an original test level or whatever, the whole game's a dream she had about someone who could leave her room, and eventually find her? But she was worried she would learn the deep dark secrets (Uboa) so she hid herself within the dream world?
Poniko explored the worlds, but was lonely with nobody, so she imagined a friend, but didn't know how to have friends after being alone for so long, so she locked herself away, and now when the friend comes she is afraid to scare her or make her go away because of everything built up inside that may come out any given time the lights go off. Of course it could also work the other way around, Mado wanting a friend - but even that person didn't like her. I'd like to believe Poniko's just a person like Madotsuki. Maybe the same kind of plague in some theories is causing people to share dreams? Instead of figments of imagination, everyone's caught up in the same universe. Even the less important characters could represent deteriorated versions of eachother. What if Poniko is just Mado, but from a different angle so that Mado sees herself the way the world views her? She may have changed her appearance to separate herself, or she could have split personalities, which the blonde and long hair could attribute to as well. Maybe I'm just really overthinking everything but I think Poniko's character deserves to be fleshed out more.