I died in my dreams once, and my dream world turned all blue and desolate like in Ghost World. That was well before I played Yume Nikki.
I know it's stupid to assume everyone experiences a similar thing, but I think Mado just imagines herself to be dead after interacting with the white flowers. It makes sense to me that she associates the flowers with death, as white is the color of death in East Asian countries.
But nice work pointing out that the entrance to the white flower field has the same shape from the outside.
I think she simply tries to imagine what the world looks like to a dead person.
In my opinion, the sky garden represents heavan to madotsuki, how she wishes what it would be like for her after her death. However, when she thinks about it more seriously she begins to feel like her death will be a desolute end evem from her worldy troubles, like she can never escape them.
Especially since you have to go through the desert to get there which houses the toriningen party, the barracks, and other depressing places of the like, then you ascend a large flight of stairs to get there. It is also possible to see someone falling as you ascend, perhaps alluding to the idea that someone wanta to escape the Sky Garden.
Maybe the Ghost world could be showing Madotsuki's anxiety and fear? Seeing as how the fleeing ghost is found here.