Well the hairbow is not even Gaga's most iconic hair but okay.
Amy's hair is more iconic, especially because she never changed it. But it's more of a negative iconic since it was widely made fun of more than "OMG I WANT THAT HAIR" - kind of like Britney's shaved head.
But Amy's hair inspired Karl Lagerfeld's hair style for his Pre-Fall 2008 Chanel show.
And if you can inspire Karl to use your hairstyle in his show, that in itself proves how iconic it is.