Hi Pratik,
It looks like the comp or layer you are using for your flag’s color map isn’t high resolution enough. If you are exporting at 5760×1080 then your comp should be 5760×1080 and the layer you have 3D Flag applied to should also be 5760×1080. Also, the map you are applying to the flag should also be at least 5760×1080.
It’s best not to apply the Flag plug-in to the layer that is the color map. The plug-in should be applied to a solid that is Comp size.
If it still looks blurry, send your project file to support (at) zaxwerks.com. When using the OpenGL renderer there is a max map size inside the materials editor that can down-res your maps and might need to be increased.
-Zax