I’m very interested in decades and am a huge fan of this show, so I’m going to give this my best shot and would love your input:

Lindsay: Likely romanticized the 50s before she “changed” (we know at the start of the series that she has more recently changed due to her experience with her grandmother. I’m guessing 50s based upon how she dresses and does her hair when she goes back to being a Mathlete in one episode, but also because Lindsay, Millie, and all of the Freaks with the exception of Daniel should have been born in 1964 and 50s romanticization would have been very common when they were growing up in the 70s.) Likely more like mid-late 60s by the end of the series - Grateful Dead interest, would likely wish she’d lived through an era of such immense change and that she’d been old enough to participate in protests for women’s rights and go to Woodstock. We don’t know how she’d have changed as she’d grown older, but I think by the time we see her at the end of the series mid-late 60s are probably the era she romanticizes.

Millie: 50s. She would idealize the 50s housewife aesthetic and strikes me as being someone who would like the fashion.

Harold and Jean are probably just nostalgic over the 50s, Jean seems like she’d miss her childhood which was certainly before the 50s (likely either 30s or 40s.)

Neal: Commentators in another thread pointed out there is evidence of him romanticizing the 20s which I’d never noticed.

Sam: 1966 born, unsure. Same for Bill.

Cindy: 50s most likely, but I could see her liking early 60s fashion.

I don’t know for the Freaks.