Yes, I'd think there is a good market for sun bonnets for adults too. I took apart my Grandma's old bonnet and made DD and sister one each from old, old fabric. Had enough to make Sister an apron to match, and she said at the Civil War re-inactment she was approached several times, someone wanting to buy them from her. Grandma's bonnet has two sets of ties on it, never noticed one like that before.
I used an old, very large dress found in an estate sale or maybe it was a yard sale, too long ago to remember. It was old, faded brown with tiny reddish roses in it. DD's was fabric I'd bleached the life out of to make it look old.
Your little sun bonnet is precious, and I'm sure it would go over well in hot weather. It would be a necessity then, and at craft shows might be a winner.