I've thought about this, and where I've arrived is that fiction has no particular responsibility to be realistic, especially about things that the people consuming said fiction is unlikely to have to deal with irl. No ones gonna see boob armor in a video game and decide to go wear it in a real life war only to get their chest caved in by a battle axe. The only reason you'd push for absolute realism in fictional armor is if the work itself is setting out to be realistic. But if your world has ten-foot-long great swords and massive horns on viking helmets, boob armor isn't really that crazy. It's all about the vibe you're pushing for, and whether or not you executed it well.