But, the thing about Nuetral Evil is, that it's typically just the evil of ones nature or creation. Ie: Xenomorphs, are a product of nature. Alien nature, but nature none the less. They are evil, but becouse thats how they survive, kill, devour, procreate. Or the terminator is an apt description of what one typically see's in Nuetral Evil. It is a atomiton of destruction, enacting it's master (Skynet's) commands without any complaint or moral imperative. Though, creatures of thought and personality can be Neutral Evil. To put it short, it's evil without reason.
Smuag can be Chaotic Neutral easily, because of his greed. It is an apt choice for any dragon, for they care only for their Horde. Their treasure. But, he takes to much joy in killing to be Neutral Evil, since Neutral evil is simply by my understanding. Evil because it is whom they are, it is their nature. Chaotic Evil I think may be apt for Smuag. Though he is not one to want the world to burn, and does not openly destroy without cause or reason. (Greed, or taking out possible allies to the dwarvs IE:The town. Or toying with his pray Bilbo, before inevitably seeking to eat him, because Bilbo's a dirty thief in his eyes.) He does take a perverse joy in his actions. A Neutral Evil character would simply kill Bilbo, but a Chaotic Evil one would toy with him.
I say he could very well be a more well worded and patient Chaotic evil, though Chaotic Nuetral also works. Since he is about himself, and though a Chaotic Neutral alignment is Neutral it can be on a side towards good or a side towards self interest. Most Chaotic Neutral characters in my opinion are conflicted between these two thoughts. The right thing, or out for myself? With Smuag he can easily be described as the darker nature of Chaotic Neutral. Sleeping on his horde, until some one gives him a reason to destroy them and all they care for. After all, he is the embodiment of self interest.