Can you still acquire Handsome Jack's mask in Borderlands 2 if you didn't the first time you killed him?

Yes, you can still acquire Handsome Jack's mask in Borderlands 2 even if you didn't get it during your first encounter. After defeating Handsome Jack, you can revisit the location and find his mask in the loot pool. Additionally, if you're playing in True Vault Hunter Mode or Ultimate Vault Hunter Mode, the mask can drop from Jack during subsequent encounters. Just keep defeating him until it drops!