Is it my acne or my bad posture that's causing severe neck pain?

Maybe posture. Acne does not cause neck pain. Keeping the neck in an awkward position or in one position a long time may lead to neck discomfort. Stretching and strengthening the neck muscles may help.
Probably posture. Acne may cause localized pain, but usually not neck pain. Posture issues can definitely contribute to neck pain. When a person slouches forward, as in someone with poor posture, the neck has to go into a more extended position to keep the head looking forward, and this can definitely increase neck pain. Thank you for the question.
Posture. I would vote forcposture achne causes skin pain and tenderness most would not describe not this as a neckache.