Digging pond - dirt and grass

My son has begun digging me a pond and without knowing better dug up
the grass and just threw it and the dirt to the side on top of existing
lawn (I had carefully removed the grass and composted it and then moved
the dirt elsewhere when I had dug my ponds). So my question is - has
anyone else just piled the excess dirt and grass on top of the grass
next to the pond and left that as a hill. If so, what were the
consequences? Did the grass just compost away after you put plants in
the hill of soil or what?

DK