I'm not so sure. I feel like the quality of fast food ingredients has gone up since the 90s.
I have a couple of theories: I think consumer preferences have pushed in that direction (e.g. I remember McDonalds chicken nuggets used to have chewy gristle in them), and JIT inventories mean that ingredients can be fresher, and food science (e.g. additives and preservatives) has improved, and improved flavor along with it, and there are also new preparation methods which improve quality of the final product.
> used to have chewy gristle in them
Chickens have gained significant breast mass in the last few decades (so much so it causes "woody breast") and, with increased supply, the price of breast meat (less gristle) has come down proportionally. Like the McRib, economics drives McDonald's decisions.
> I feel like the quality of fast food ingredients has gone up since the 90s.
Interesting. My impression has been the exact opposite of that. Ingredients seem more uniform, true, but they also are much less flavorful and seem to be higher in salt than ever.