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.
I don't think it's an issue that they'll be deflected towards the heart, no arrow, spear, sword tip or hammer's spike will pierce the heart through a decent breastplate.
The best I've ever seen accomplished is denting or a couple centimeters of penetration. That wouldn't be likely to happen in the center of a breasts' plate as armorers as far as I understand always tend to leave areas like that fairly thick.
I might be underappreciating the metallurgy that happens in a crease like that, and the amount of stress it'll be put under with so many strikes directed to a center crease.
The main issue I see is if something is deflected up towards the neck. There's usually a "V"-shaped ridge to prevent that, even in this illustration. But I'm thinking that a couple domes offer a lot of variation and maybe opportunities for something sharp to bounce past the neck guard and slide right into the gap. You should likely have a thick collar and some mail there if your armor includes a mail coif (hood), but those are a bit less protective than the plate. Good mail is still pretty hard to get through, though. But getting a few centimeters of a spear through is enough, and that's not atypical from the tests I've seen. I'm guessing even just the impact can crush your throat and mess you up, even if the mail manages to withstand the thrust.
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.
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