I'm gonna level with you - I actually really like how the beauty mechanics play out. You have the lab procedure, or beauty potions (which last a long time) or clothing to affect it, so there's quite a bit of flexibility. But the fact that Beauty can only be changed to a point makes higher grade captures that much more valuable, because grade is what determines the base beauty. It doesn't matter if you're going to bust them down to Slave grade - if Beauty is important, you'll probably want to catch a Noble.
I get where you're coming from - so much else is malleable, and it's frustrating to be left to RNG for something that's sometimes critically important (I'm looking at you, Sweet Voiced Nereid). But in a way, I kinda like it.