Where does the descriptor show-up for how thick or thin your thighs are?
Any time the [pc.thigh] parser is called (or its plural, [pc.thighs]); if you qualify for any of the thick descriptors, there's a one in three chance that one of them at random will be added to the output (though some instances force an adjective to be added if possible, so it's guaranteed in those instances). There are no descriptors for thinness.
[pc.thighs] can actually be called in the Appearance tab under certain conditions; specifically, having lapine, quad-lapine or leithan ears with the Floppy flag and a length over 50%, but no more than 100%, of your height, or a 1 in 2 chance if you have a hip rating of at least 25 and legs.
My question is are we going to see a TF that would allow the player to change their thigh ratios or will that only be a thing that the TiTs editor will do (if that option is added in the future)?
I doubt it. There's no code in place to influence thigh size independent of hips & thickness, and it's probably not worth the effort to put such code in place.