Gen z are the millennial's kids. The older millennials are hitting their forties now. Not all gen z are the kids of millennials, some are gen-x kids too.
I've never seen any kind of outrage over comfortable clothes by millennials on any big scale. Or gen-x. Hell, not even all of the baby boomers do, unless it's at work.
If you're seeing it irl, then it's likely just parents parenting, which is a different thing entirely. Online? You gotta at least grab some screen shots of handful of examples before this is even believable as a generation level thing. Better, provide links to it, since screen shots can be faked easily by anyone older than about 12.
If you are seeing it, is it on a specific media outlet? You'll find that some are more prone to stupidity like bitching about other people's clothing than other forms of media.
Tbh though, how the fuck can you even tell what generation the people complaining are? Do they give their age? Seems pretty damn weird unless it's YouTube, but most of those are memes to begin with. "I'm 70 years old and I love/hate this" type of bullshit.
I think you're full of malarkey tbh, but if you aren't,I would actually be interested in seeing where this is because millennials have rocked comfy clothes in public for well over a decade. It would be very funny if that's changing as they age.