Car cover may help reduce the cabin temperature from going far too much above ambient, but I don't see how it'd help with the battery. There's no IR path to increase the battery temperature from the sun, so only convection and conduction would apply. There's little conduction, so it'd be mostly convection. Far bigger source of heat will be convection coming from the black-top asphalt, and car cover would do little to help.
Things that may help are parking under a shade, parking in light colored top (ie, grey concrete or grass), pick breezy areas. But none of these will get the temperature below ambient.