Hi guys, this image show everything on EPIC and materials on HIGH. Why is still my preview so bad like on picture?
idk but i will try to help you 
, try using PostProcessVolume and adjust it a bit
Anything materials-related is largely built on the material-shaders tied to the mesh. Just turning stuff up to high/epic more effects draw-distance, quality-of-lighting, and on lower-levels, lowers the screen-percentage used to generate the raw-image (does upsampling).
If you want more than reading-out textures to basecolor, specular, etc, which is what a lot of sample/stock materials are, then you will be required to put in some legwork. In this case, what you put in really does drive what you get on the other end.
Best I can suggest is that stock-materials, esp from marketplace assets tend to be, well not bare-bones, but ‘serviceable’. Things like UltraDynamicSky and the like which have a lot of work done, tend to be a standout.
I’ve always understood that meshes, that kind of assets is more the mesh and the expectation was always that I was going to need to do work on the shaders to bring them in-line with whatever running-spec I decided upon.
What assets are you using? Can you show us the shader complexity (alt-8 in the editor-perspective-view)? I’m just curious what the expense is. If it’s on the green-side that means the materials are light in expense, which means mathwise, they won’t be doing as much as other things that might do transparency, reflections, etc.
Past that, just for color, as noted by the above-post, you could use a post-process volume to adjust quite a few things, but that wouldn’t add in reflections, transparency, or other effects; it will just be color and some other stuff…unless you write a post-process-shader yourself and we’re back to making-materials… 