I'm wondering: are the people who don't like it and hate the costumes those who were born in the '80s or later?
Doubt it, the episode had a bad rep in the 70s. I've always thought that the people who hate it the most are the people who have an accurate recollection of hippies.
I personally think that the wincing discomfort we feel at the space hippies, works
in the episode because the senior officers feel it too! It may not have been intended as such, but it works as a storytelling device. Also, theoretically they're a couple hundred years removed: they could be expected to fail at getting the details exactly right, if they are imitating 1960s hippies. And if they're not, then there's even less grounds for complaint about them.