To everyone who's trying to figure out why would Amon, go through the trouble of putting a mask over Aang's face.
The Avatar. is the greatest bender to ever live, and Aang was one of the greatest Avatar's to ever live. He stopped a war that killed his people and almost destroyed the world.
He brought peace to a world scared by said war, he brought the divided nations together as one and created the United Republic of Nations, the greatest achievement the world had ever known.
But, it was all done by and for benders, despite how hard Aang tried to make it for everyone benders and non-benders. Putting Amon's mask on Aang's statue would be a final, and great insult to the Avatar and bending it's self. That's why, it should be known that honour is a very important thing in the Avatar world.
Amon insulted Aang's and the Avatar's honour by doing this.
That is why Amon did it. To destroy Aang's rep and honour and that of the Avatar and that of all benders,
And yet another bizarre quirk/flaw of Book 1's finale rears its ugly head. ^_^; I personally found the defemation of the statue to be a little too convoluted and silly, personally. So this made me smirk.