Primitives isn't the right word for the Mesoamericans. They were a socially advanced society; their circumstances just didn't lead them to develop the large scale military hardware and techniques that Europe did.
I would assume they did loot guns and horses from their defeated adversaries, yes. The only scenario in which that makes you more and more competitive is when you acquire comparable numbers of said resources, and can use them as effectively as an army that has been trained in their use. I don't think either is the case, but this is just me hypothesising because I only know the high level details of the conquest.
