Ah. Should have looked there.
No, I don't think the monk should get that. Monks do have natural weapon, but they have more than one. While the feat is clear to use for a dragon (increasing his bite damage), you don't know what weapon the monk uses, and he's assumed to use whatever he wants.
Note that Greater Magic Fang will increase one natural attack by 1/4 HD or all natural attacks by 1 - this seems to be the monk option. If a monk is to benefit from Improved Natural Weapon, he should probably take it several times - which makes the feat useless to monks, IMO (they're opportunists, not destroyers).