An image has pixel. A 3D model has polygons.
The amount of either one determines the amount of details and quality you have in a visual. A polygon can be a 3 or 4 edge shape which, bayed one at another, creates a 3-dimensional shape. Now while this does not directly affect your cloth simulation it defines the amount of shapes/triangles you have within your 3D model, hence the number of details and quality.
Usually you can define the polygon count within your software as well as how it is output. It will affect the quality of your render but also the render time and performance of your software when you are working on your model.
Changing the polygon count in grid view.
Also for other aspects than your garment’s shape geometry plays a major role, i.e. trims. An image of a button looks much realistic than a 3-dimensional button. Basically for any physical piece of your garment you ideally want to have a geometry than a a flat image (which is partly more common and also easier to handle in the 3D space) as it adds tremendously to realism. See here.
Polygondressing 
