Focal length has nothing at all to do with 'bokeh'. "Bokeh" is the subjective quality of the background blur, not the amount of blur.
But with respect to perspective (ie. flattening at 200mm vs. 'distortion' at 24mm), focal length has nothing to do with it. It is 100% due to the change in subject-to-lens distance. A shot with a 200mm lens will look exactly the same with respect to perspective as a 10x crop of a shot with a 20mm lens. Exactly the same.
Now of course, the 200mm shot will have more resolution and better IQ by virtue of not needing to crop it. In that respect, perhaps there is a 'better' and 'worse' focal length for portraits; that is, the focal length that allows you to achieve the subject-to-lens distance that gives you the desired perspective.