Kinda weird that a simple action like riding over it with a bike can pull out all of its hair. If that was a car or at least a motorbike and its hair stuck to the spokes of the wheel, that may be possible, but this is a bike, which is unlikely for that shitbag to lose all of its hair just by standing before the bike.
I find it ironic that after bailing out on her kids, in death the deibu managed to provide some comfort to her daughter, with her ribbon keeping her slightly apart from the corpses.
As for her hair, odds are the deibu's been down there for a while, and lost the hair due to rot, similar to the Marisa in the previous story.
The offensive death stench and rotten stench paralyzed Reimyu's senses.
Reimyu continued on crying sadly.
She didn't notice the Reimu she was riding was her mother Reimu, who was run over by a bicycle and thrown here right after abandoning her.
To escape from the damped, wet feeling of her foot, Reimyu took refuge by ascending to the accessory of an adult Reimu's dead body nearby.
"Yugu...
Shcawwy...
Why ish Reimyu like thishhhh...""Yunyaaa...
[Reimyu'sh] hungry...
Why hash princhessh Reimyu...
Become hungry like thish..."