Related – How does one dispatch a helpless opponent?
Here’s the unconscious condition description:
An unconscious creature is Incapacitated, can’t move or speak, and is unaware of its surroundings The creature drops whatever it’s holding and falls Prone. The creature automatically fails Strength and Dexterity saving throws. Attack rolls against the creature have advantage. Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.
So attacks against the creature have advantage, and any attack that hits is a critical – but you still can miss because of the creature’s AC. Armor Class includes dexterity bonus. The description doesn’t say the creature’s AC changes somehow.
Does it mean the creature still benefits from its dexterity, both in terms of mechanics and in-game world?
An example situation
A low-level party of Barbarian and Bard fights a sneaky thief, who has AC of 15 due to his +4 dexterity bonus. The Barbarian attacks, so does the Bard, but their results are 12 and 13. DM described that the thief was twisty enough to dodge both attacks.
Next round, the Bard puts the thief to sleep (hence, unconscious) with the Sleep spell. The barbarian makes a melee attack with advantage, but his best result is 14. It is still a miss, isn’t it? As a DM, how can I plausibly describe such an outcome?