1, "doesn't give a shit about defense" so that's the explanation for the lack of armor right? I'm not surprised a demon wouldn't worry about protecting organs they might not even have.
2, is the boob shape on the chestplate why it doesn't get the full 10? Would you rather the chestplate be flat and be very uncomfortable for her boobs?
3, is there a reason the wings are coming from the demon's lower back?
Chest plates aren't meant to be flat. They're meant to be round. The reason is that rounded armor is structurally more stable and it's easier to get a weapon to deflect off rounded armor than a flat armor. There should be plenty of space for breasts in regular medieval armor without needing any sort of modification.
Having the chest plate fit the breasts actually introduces a point of weakness, since glancing blows will be deflected toward the heart.
I watched a video where someone tried out boob shaped armour- the problem wasn't the deflecting IIRC, it was that the armour itself came to something like a point against the chest so any impact HURT.
The problem with boob plate is how it deflects attacks. There are plenty of places on that armor where a spear jab would deflect upwards toward the neck area.
Flat isn't really what you want either for these. When the weapon deflects and slides off to a side, that steals a lot of the energy from the impact. If the weapon strikes a flat armor, it would be much more likely to damage the armor. That might mean caving in the plate, or fully piercing it. Neither is great. (And even if the armor withstands the hit, it might knock you on your ass, which could very well be a death sentence)
There's usually some room in these sorts of armors. unless you're tiddy is comically big, I'm skeptical that you'd need a great deal of accommodation. But even if you do, the armor shouldn't look like booba from the outside.
Something like this should presumably work, but I'll admit I've never had boobs inside of a suit of armor.
1, the demon is wearing armor but not on their body. Deciding that their extremities are of need of armor but not their core is a pretty half assed worldbuilding excuse to make a hot demon character who shows off her boobs even though it's unlikely to be practical for combat
With respect to number 2 I assume they're talking about the detail added to the top of the breastplate to imply the boobs, and the sculpted shape where you can see the boobs separately, not the fact that there's enough room for boobs. And sculpted boob shaped breastplates are generally not super effective as you want a shape that swords or other weapons glance off of, unlike concavities between the breasts which would guide weapons to a point where force could be concentrated.
3, I assume that's mostly so it can fit in the composition, or is easier to draw without overlapping the raised arms, but yeah it's a bit odd
I think it's perfectly fine to like fantasy stuff with loose world-building and costume design based on whatever is cool or fun or hot, but it's just as valid for folks like the person who made this to want costume design that's grounded and doesn't exclusively sexualize every female character
I know butt wings are a thing in some Japanese works, it's mostly for fanservice as best I can tell. I don't know where or when the concept originated.