Unarmored AC is 10+ DEX modifier so then if you wear leather armor, it's 11+ DEX modifier. I can read that to mean you don't need to wear both clothes and leather armor. But clothing is intuitively better than nude, so it does need to do something.
Since we are tracking clothing for immersion, as with everything, it must have a purpose. So I am going to assume that any armor's AC is reduced by 1 if you don't wear clothes and/or undergarments. I would agree it makes little sense against a sword if you're wearing plate armor and arguable if the clothing is doing anything; however, everything is averaged for simplicity, so imagine fire damage with plate armor and tell me it would be the same if you are wearing clothes under that metal or not.
So a robe, undergarments, children's under ware, and all other variants of clothing that is not armor will be counted as that 1 AC, so then that leaves nude. A nude humanoid would them have AC of 9+DEX modifier.
