Whew. This was fun to do, but it took such a long time @_@ And you can barely see all the details anyway T_T I'll try and point them all out as best as I can:
First off, I don't like to think that Poniko's transformation is voluntary, but rather something akin to lycanthropy or when a person with bipolar disorder goes into a manic or depressive phase; it's something that she can't control, but she nevertheless has to endure it. Why this transformation is so drastic is simply because this is all a dream. With that in mind, the text in the background is made to resemble any frustration she might have because of her "condition", including isolating herself in a hard-to-reach place so she won't harm anyone. Even when Madotsuki makes it to her house, she still ignores her, hoping that she'll stop trying to reach out with her and leave, rather than staying longer than she should and finding about her secret. The "あ"s in the background represent her screams of agony/frustration. "助けて助けてお願い助けて" (I forgot what the last part meant ) translates as "Please please help me please" (According to Google Translate). "の貴重な友人を許してください" says "Please forgive me, precious friend" [Madotsuki] (Please forgive valuable friends, according to GT XD). "馬鹿だなぁ私は" means "What a fool I am"; "なぜ誰も来ないのだろうか？" says "Why will no one come?" [to help her], and lastly, "さようなら..." says "goodbye...", referencing her giving in and transforming.
A screenshot of Poniko from the game is on the top of the picture, and an upside-down game screenshot of Uboa is on the bottom. Her house is bookended in the middle.
On Poniko's screenshot, you can see light red halftone dots (trickling blood) and grey dots up Uboa's (Being consumed by darkness)
Uboa is also outlined in the field of all the "あ"s
So basically, a whole picture spawned on a lone (and somewhat convoluted/weak) theory I just randomly came up with XD I hope you like it just the same ^^
Yume Nikki © Kikiyama
Done on Manga Studio Debut 4