it does insulate but at the same time it also helps maintain a stable temperature for the fermentation in the soil and amongst the mulch

one thing i can mention is that if you add sources of nitrogen to a pile of mulch it makes it cook hotter and faster (for example, stable litter or rabbet litter or similar, or guano, or just peeing on it) but yes the pile itself helps keep the temperature in optimal range

if you have a big problem with sun heat you can put almost anything on top to reduce that a bit, though black is probably not so effective