Oh man I was totally going to lodge a protest if Yuno didn't make it into the example images somehow. But there she was in the last one, yay.
X! <3
It looks like most aren't tagged, and personally I wouldn't be inclined to do so myself in most cases. But if you do it looks like (putting aside the popular ribbons and bows) by far the most used tag is just hair_ornament. It's suitably broad enough to capture all of those examples.
"Any picture with ornaments on the hair. A hair ornament is considered to be any accessory to stylize the hair."
Seems good enough to me, honestly.
