Was the situation complicated? From the narrative's perspective, The Joker needed to get away from Arkham. The writers could have had him jump out of a window, or have used the old cliche of hiding in the laundry. Neither of those things would have required a great deal more thought than the Christmas tree.
We are also overlooking the fact that it was a Christmas themed episode. Does that not invite a bit of seasonal buffoonery?
It´s not supposed to be easy to escape from AA. Making it that way is, by itself, a poor writing decision.