Is There A Way To Display My 2D Texture Flats?

This is not possible with Marmoset viewer. We feel 3D presentation is the most intuitive way to display textures. Additionally, 3D texture presentation allows us to show texture content regardless of how many materials or maps were used to author the materials.

If you want to show your 2D flats, you can include traditional images on your portfolio or ArtStation page.