Episode 4 in season 1 of Law & Order: SVU is about a policeman that kills prostitutes, and one night he killed a woman that was in good health and had good nutrition, and the mortician referred to her as a prostitute from the 90's. Why?
The M.E.: "Stomach contents include soybeans; vegetables; whole grains. A health conscious hooker."
Olivia: "How 90's."
Saying that a health conscious hooker is so 90's in the 1990's would be like saying, "How millennial." in 2010. I'm not sure what it is now. The year isn't important; it's just a saying. Every decade has had its health kicks; coffee and eggs seem to have been continually debated throughout the last 100 years. Renewable energy? How 70's.
The 90's were a strange convergence of opportunity and availability, and could leave you with a feeling of we can do anything and we can do better. See The Dot Com Bubble to learn how that ended. AFAIK, the 90's were also the first decade in America that the trend to become health conscious became trendy.
So, how 90's of that hooker it was to be concerned with their food choices, while they subjected their bodies to prostitution.
In retrospect, I can't rule out the chance that Olivia might have meant something more along the lines of, "Kids these days...", but I still think the comment is more about a sign of the times.