The idea is fine, the theme usage is pretty valid as well.
The major problem I see is that you can cheese all the levels by picking one box and dragging it along the player to get it all the way from the start to the end of the level. I think it would be nicer if you have some more elements and some restrictions when making the player move.
I understand that the time limit was a important factor, so for that, I still think this was a pretty good result.