No, it comes from the outer container which is layout-card.
Place 2 Entities card in a vertical stack - you will see a similar margin.
And it is supposed to be changed on the container’s level.
So, you are right in part that it cannot be styled from a mushroom.
So:
– check native layout-card options to style a margin;
– if not - you’ll have to go from stack-in-card level…