It's a very strange look... her clothes aren't just manly, they're way oversized. I've read that she made her own clothes for quite a while, so I think maybe it's just a combination of that, the masculine look of them, and also perhaps at that time period clothes were less form fitting than they are now.
She seemed to be pretty careful with her image, so I would imagine she wanted to appear that way... so it's either just a style I'm ignorant of because it's so antiquated or perhaps she was trying to look more like Lindbergh? She looks uncannily like him.