I have a tame alpaca that died on its own (of hunger I think). Can I butcher it for meat and wool?
Nope. Dwarfs are superstitious, and will only butcher dead wild animals or tame animals that they slaughter themselves via the animals screen.
And by superstitious, I mean, "It's a quirk of the game, don't think too hard, please".
But yes, Alpaccas (and all grass-eaters) can die of starvation; make sure you're pasturing them in a grassy place so they don't starve!