Most EVs will both heat and cool their batteries as needed when plugged in to maintain a healthy temperature.
Insulation also works to keep heat out.
Or in – a battery while charging tends to emit quite a lot of heat on its own, doesn't it?
Not if the battery itself is generating heat. Then the insulation keeps the heat in.
Doing this in a passive way would put significant upper limits on charging speed. You'd have to combine insulation with some active way of getting rid of excess heat.