the difficulty in implementing that is that the hearthlings mine from their “feet” location, so they have to be standing on or in front of the block they want to mine. They can go 2 blocks above their feet level, but only if they’re standing directly against that block they want to mine.
To fix this situation, the hearthlings would need to figure out whether mining a given block would make another block within the order inaccessible… for a small area that wouldn’t be too bad, but for a large area that would be an insane resource drain on your CPU/RAM.
I think that a simpler solution would be to have the hearthlings simply build ladders on a block-by-block basis whenever you give them an order to work on something they can’t reach. They already do it with building, so it would be intuitive for players that they’d reverse the process rather than players having to manually build those ladders.
Of course, I have no idea how difficult it would be to make the game figure out when a ladder is required for mining; but I would guess that it’s easier to go that way than to have the hearthlings “pre-simulate” the mining task for every block to figure out if they can accomplish it.