I've done some experiments with mycelium as a construction material, but I'm hardly an expert. Mycelium weighs anywhere between 50 and 950kg/m3. Usually you won't have mycelium as thin as cardboard, because you want use mycelium as a 3d buffer, replacing styrofoam. EPS (styrofoam) has densities of 15-30kg/m3. So while it's more sustainable it's also heavier.