I imagine there's a bit of a correlation.

One, being smoking hot takes time and effort (hair, makeup, skincare, fitness, etc.), and some inherent degree of vanity. There's nothing wrong with taking care of yourself and taking a basic level of pride and dignity in your appearance, but at a certain level it becomes obsessive.

Two, even if you're just naturally hot, you were probably always very attractive. People tend to be nicer to hot people and tolerate their bullshit way more. Without the natural pushback of being called out for emotional overstepping, there's no pressure for maturation. It's kinda like the trees grown in that biodome that collapsed under their own weight, because there wasn't any wind to strain and strengthen them.

I'm definitely not saying hot = emotionally unstable, I know plenty of very attractive and stable women. It's not universal. But the most self-obsessed and emotionally volatile women I've known have usually been very hot.