Separating out the ethical issues, in order focus solely on the question of 'tacky,' I don't associate Primark, or any other cheap brands/shops with being tacky, because they're not pretending to be anything other than what they are. They are brands selling cheap clothes, enabling people to get a version of what's fashionable at any given moment, for a small price tag.
Likewise, I think it's slightly odd to say that a brand like Next is tacky. It's just a relatively basic, generally reliable brand for kids' leggings, tee shirts, work shoes, whatever.
To me, 'tacky' means inauthentic. Cheap crap being sold as 'high end,' relatively poor quality (C Tilbury, Yankee Candle etc, for example) things being flogged as good quality. That's just how I probably view the general concept of tacky though - I would never extend that to judging what other people wear/want.
to sit in a circle and discuss what is tacky- I just don't think it's a classy thing to do. I think you have to examine what it is you mean by tacky, and whether it's something to do with social class, income, or something else. Good-hearted people just don't look at someone's clothes and make judgements about them. It just isn't done
I probably agree with this.