Okay, quick explanation on how the customisation system works on a base level. The customisation system has two major parts, one being mesh deformation (used to change the shape of the naked body to match the shape of clothing) and the other being additional meshes, these are things like a lot of piercings, belts, holsters, the bottoms of skirts and some head gear.
All things that deform the base shape of the body have no impact on texture resolution. Additional meshes however almost always need additional texture space. Now by default there are 2 "tiles" of texture space on the character texture free for extra meshes. This means that you can almost always use at least 2 extra meshes without having to worry about the resolution being lowered by the system. If you add another mesh, one of the existing meshes will have it's texture resolution halved to make space for up to 3 extra additional meshes, all at half resolution. Eventually it will also half the resolution of the eyes, feet and teeth.
Now I don't think I have seen the layout of the headphones, but it is possible that it might use both free slots, and when you add the skirt, it needs to scale one down, and it picks the skirt. The real issue here is that there does not seem to be a way to manually control which mesh gets a reduced texture, and in this case it would be much better for it to pick the headphones.
The complexity bar is only there to stop people from killing the texture baking process with too many layers and has nothing to do with any of this.
TL;DR: reducing polygons will do nothing, this is a limitation of the system that will not go away.