Sorry if I’m ignorant but this is my opinion…
Firstly, the song is a Japan only single (as far as I am aware) and Avril has taken the opportunity to show her appreciation to all of her Japanese fans by filming a video there and using some Japanese language in one of her songs.
How is the song racist? It’s not about Japanese culture and it’s not disrespecting it. (There are countless J-pop artists that use English phrases in their songs so surely it’s not the language you’re referring to?)
Avril has not made a song about Japanese culture, it’s about a sleepover/having fun/her love of Hello Kitty
That’s where the idea came from - Hello Kitty is Japanese, Avril loves Japan and has many fans there - she often says “Minna saiko, arigato” at her concerts in Japan/that’s where she learnt it
The “Kawaii” is referring to Hello Kitty, she thinks Hello Kitty is kawaii and she is right!
All in all, I understand why parts of the video may be a bit problematic and easily misunderstood but Avril is in no way trying to be Japanese, making fun of the culture etc.
As you say Japanese culture is not “all about being kaw[a]ii” so how is it cultural appropriation? In the video she is dressed as Avril Lavigne and she is herself, having fun in Japan.