The waist in my opinion isn’t as much of a factor as the turning radius. I got a 98 mm waist ski with a tight turning radius and it kills bumps way more than my 85 waist with a wider radius.
flex is going to be the most important factor when skiing bumps. softer skis will be less stable and more unpredictable. Stiffer skis will do the opposite, and be less nimble.