Find the shortcut on your desktop & open its properties by right clicking it & selecting "Properties". Make sure you're in the "Web Document" tab.

Select "Change Icon..." (if you don't see this double check that you're in the "Web Document" tab). From here select "Browse..." & navigate to [.../Steam/steamapps/common/Sonic Mania]. Select "SonicMania.exe" & click "Open" now select "OK" followed by "Apply" & finally "OK" to exit "Sonic Mania Properties".

If you followed this guide correctly you should now see that your game shortcut has a much higher-res icon then before.