So, the question is if the knowledge about fat viscosity could help design diets to prevent or treat diseases and why is not more research about this.
There is an ongoing research; I don't know how much USDA is involved, though.
There is some evidence that the fatty acids with 4 or more unsaturated bonds (arachidonic acid, EPA and DHA) improve fluidity of the cell membranes, and that the diets high in these fatty acids could, theoretically, help to prevent against Alzheimer's disease (PubMed, 2010).
Some short-term clinical trials also suggest that DHA from fish or supplements may help to prevent Alzheimer's disease and one of the proposed mechanisms is increased fluidity of the brain cell membranes (PubMed, 2010). On the other hand, a 2012 Cochrane reiew has not found any association between high intake of fish oil (high in DHA) and dementia.
According to a 2018 Cochrane review, "taking omega‐3 capsules does not reduce heart disease, stroke or death. There is little evidence of effects of eating fish."
The problem with studies about nutrients effects on health is that they need to be long-term (>10 years) and include a lot of participants to be convincing. Many studies about nutrients have been disappointing with no beneficial effects shown, which may discourage some researchers to start new studies.
I can make another answer about antioxidants if you ask a separate question.
The questions only USDA can answer can be asked here: