It appears that Ao-chan herself has accepted this as part of her image. In a recent magazine feature, she includes her own drawing of herself, emphasizing the eyebrows (right).
This is one in a series called "Aoi's a-i-u-e-o." "A-i-u-e-o" are the first five letters of the Japanese syllabic alphabet. This particular week's feature was based on the letter "su," the first letter in "supootsu" (sports), and includes photos of her dressed for tennis (see bottom of page). She has said before that sports is not her thing.
Now, her eyebrows aren't all that prominent, as you will see from the photos that follow. But they are sufficiently prominent that she almost always wears her bangs long enough to cover them.
In their fairly gross way, some 2channel posters have speculated on the amount of hair elsewhere on her body.
Anyway, here are some photos showing her eyebrows:
|When she was nine, on Appare Sanma Dai Sensei|
|Aged about 14, from her old blog|
|In a publicity message for Yumeiro Patissiere|
|Cosplaying the ED for Soremachi|
|With Hirakawa Daisuke, her co-star in Hyakka Ryouran|
|(click this and some of the other photos to enlarge)|