Katy Perry isn't even relevant to Ayu. I mean yeah they are both popular female singers but that's it.
Ayu is far above her. Katy Perry is a performer/entertainer. A product of Hollywood. Ayu is a creative artist who earned her success through hard work and a drive and passion for music.
I think the main difference from the older Ayu music to now is that her music is less adventurous. I mean Free & Easy is a very unique song. It is far from conventional. It can barely be placed in a genre. But really, ever since My Story her music has become more genre specific. Creating straightforward rock songs like About you, 1 LOVE, talkin 2 myself, Rule, as an example. It began once CREA became less involved and it shows with My Story. Then with missunderstood, it was still a little varied because of the unique Sweetbox songs but after that the music is still great but isn't as avant garde as perhaps 2000-2005 approximately. I think the quality is mostly the same but because there is less creative experimentation now, when you compare it to back then it seems like it is lacking or doesn't have the special quality.