No, the joke is that the Viet Cong often resorted to hunting 'bush meat' of local wildlife just to survive, including monkeys.

Monkey meat is a common food in Veitnam. , this and dog meat is normal in many Asian countries. It's a cultural norm for a species indigenous to the reason, kinda like every cultures food.

If you think it's wrong or immoral, I'd suggest doing some thinking on what exactly is the difference from eating cow, sheep, chicken or pig. Either all animals are fair game or they aren't. It would be hypocritical and racist to ignore or limit another cultures traditions.