How do you define "Faddish?"

This is a definition by exception. It's not what it is, it's what it is not.

It's not "classic" or "traditional."

Although a "High & Tight" is extreme, it's traditional, therefore not faddish.

Skinny Jeans & Bell bottoms (excluding Naval uniform traditions) however would be considered faddish.

Part of this is evolution of words. Words change meaning over time, so what seems like a vague word today may have been a very specific word when it was first introduced into the regulations long ago.

One word: Hipsters.

AR 670-1 gives leaders a lot (maybe too much) flexibility to enforce uniform and appearance standards. A squad leader can explain to his/her squad what he/she deems faddish, which should be nested with what the PSG deems faddish, which is nested with what the 1SG deems faddish, etc. etc. The commander is overall responsible for the discipline and good order of the unit but, NCOs enforce the standard (sometimes above what the commander deems acceptable).