Idol Song is more similar to an asethetic than
a musical genre. Idol Song usually concerns
particular lyrical topics and performers typically
wear stylized clothing. This asthetic can be put
on top of more traditional genres such as J-pop,
Kayokyoku, Metal, Ska, Euro Beat, funk, jazz, etc...
So long as a performer self-identifies as an Idol,
their music is considered Idol Song.