Do female pilots cut their hair short? How would their bun fit into anything?

The helmet requires a tight seal for hearing protection and also requires a snug fit so as to stay attached to the head in crash sequences. A flight surgeon used to “fit the helmet” in the early days, and were not suppose to be “traded around” (borrowed) due to the strict fitting requirements. All that to say, that I seriously doubt a “bun” can be worn. It would tend to create a horrible “hot spot” on the head. The few women pilots I saw years ago did not wear buns.

Female pilots don't have their hair in a bun when they're on the flightline. However, when they're not flying AR 670-1 applies. Essentially Army pilots abide by a different set of uniform standards when flying and not flying.
Flying:
No headgear
Un-bloused boots
Collars Up
No saluting
Females hair can be let down in a ponytail.
Not flying:
Same as every other soldier

No they don’t. They’re allowed to let their bun down into a ponytail when they have to wear their helmet. (In the Army anyway)