Simple idea that somehow works out pretty well.
The levels are too big in my opinion. Takes a little long to clear them at the moment.
Also it might be interesting to see some more interaction between game objects. What if monsters are slowed down by dirt? So the more dirt you eat, the harder the level gets. This could also help to make it a bit more climactic and raise the tension at the end of a level.
Moving diagonally gives you quite a speed boost at the moment, so it's clearly the best strategy. Maybe you can find a way to fix that.
In general I'd say you can be pretty happy with your result. Hope you had fun and thanks for participating. :)