To answer the question, viscosity of epoxy is critical if you do two things: 1) spray painting ; or 2) resin application. Otherwise, you're likely looking at the wrong product.
For flooring, people use typical paint roller, and viscosity (or density) isn't critical. However, to achieve a certain thickness per coat, density does matter and people called that percent% solids. The higher the percent%, the thicker (higher viscosity) it will lay to achieve the thickness per coat (unit is mil). Typically, the thicker it lays, the more durable it is.
Well... sort of...
Viscosity is the relationship between shear stress and shear rate. In simple terms, it's resistance to flow.
Density is the ratio of unit mass per unit volume of a material.
Percent solids is (usually) the weight of nonvolatile material using some defined drying conditions as a ratio to the starting weight of material. One can also define percent solids by volume than weight, but we'll stick to weight here.
While all three of these may be correlated for a given formulation, they are independent factors. One does not determine the other.
For example, one can have a coating with dispersed tiny microspheres (well known technology, in fact). These add a lot of volume but not much weight. They can affect visosity, will raise solids, but may lower coating density.
It's not automatic (to me as a coating scientist) that 100% solids coatings are by definition better than coatings with some sort of solvent added. It's much more complicated than that. Pigment to binder ratios, for example are critical to coating durability in a high wear environment and we haven't talked about that yet.
I'd rather judge coating quality by some objective measurement of the dried and/or cured coating film. Elasticity, tensile strength, abrasion resistance, sensitivity to various solvents, UV, temperature -- those kinds of things are better measures of how well a coating will hold up.
I needed something to do while I was eating lunch.