Sure they are blurry due to upscaling from a very low resolution icon
That's a shame, isn't it?
Simutrans has recently been made high DPI aware.
If you are referring to my manifest, well, Simutrans is I guess aware of high DPI, but doesn't do anything about it (except tell Windows' DPI scaling hack to *** off).
It is possible that the manifest may be extended to include new versions of icons for varying DPI and icon sizes.
That has nothing to do with the manifest. The icon just needs more images, high-res images. Currently, it has 32x32 and 64x64, both 8-bit by the looks of it. I can't find anything indicating that Windows supports vector icons, so 256x256 are the biggest they get, although what I found seems to be about Windows Vista/7.