Very nice! I like the 1-bit artstyle. And the progression of seeing all the little goblins mining more and getting further down is satisfying. I didn't realize until my goblins were still going off screen that I could use the right mouse button to drag and scroll up and down, they had a lot more to go! I think given the nature of game jams and playing jam games, it might be nice to have larger bonuses per upgrade level, so that the player can see more significant progression in a short time. Also was encountering a bug where they would all get into the same spot and not be mining anything, even though there was a block in front of them to mine.
All in all I enjoyed it a lot, and think it could make a fun full idle game, maybe they dig mines to the cores of planets and once they're excavated they move on to another one. :)