@shaderbytes, I agree with @bghgary in that your F0 should be between 0.02 and 0.05 which is where most dieletric materials fall for their F0 value. Conductors are usually higher, but this is a safe range for dielectrics.
The other thing affecting the overall light in your original image is the IoR value of 3.0. If an IoR value of 1.0 is how fast light travels in a vacuum your value of 3.0 means light is traveling through your material at a speed three times slower than in a vacuum. Diamond affects light by bouncing it around inside the material before reflecting it from the surface which slows down the light, but the IoR of diamond is only 2.417 so you have set the cotton fabric of your material to slow light even more than a diamond meaning it is bending the light as it travels through the material. While some wavelengths of light are indeed absorbed by the material, we aren’t really dealing with bending light through a cotton or poly blend fabric. Almost all liquids and solids have an IoR over 1.3 with most plastics having an IoR between 1.3 and 1.7 so unless I have a specific material that I can get a precise IoR for, I tend to leave my IoR around 1.5.
This is what the render looks like with an IoR of 1.5 and an F0 of 0.04:
To help understand what light is doing on the surface, I disabled the skybox and changed the clear color to black. Your base color is very dark, and it may be a little low for PBR (26, 26, 26), not out of the ballpark but dark enough that you won’t see a lot of detail. The other thing that is likely contributing to the issue is that your normal map is pretty crunchy meaning there is a lot of change in the perturbed surface normal in a very short distance which you can see by the drastic change in color in the normal map:
Reducing the normal strength of your texture by approximately half will help control some of the surface noise as light tries to hit the ridges of the material and horizon occlusion tries to cull light that is on surfaces rolling away from the view angle. This should give a less mottled look to the fabric as crunchy normals will make the surface feel dotted with bright and dark pixels.
Lastly, I reduced the roughness of the material to about 0.7. There is nothing in the real world with a roughness of 1.0 and most fabrics will have some sort of reflective nature in the fibers from dyes, to thread composition, to plastics or coatings in the thread. This also gives you more headroom in case you needed to show a dirty fabric that may be coated in dried mud, which would almost certainly be rougher than a cotton or poly blend fabric but would also not be a roughness of 1.0.
Lastly, @bghgary is right that baking out an ambient occlusion map will help with culling light where it is not supposed to be like under the arms. Obviously this changes if the model is animated as you need to be careful how you bake an AO map for a mesh that will move. In that case, baking it in A or T pose will likely give you the best compromise for an AO that helps but does not break the lighting on the model when the pose changes.
I can repro the issue you reported about the texture being inverted when saving out to Babylon so we will look into that.
I hope this helps but feel free to ping with more questions.