My observation as a low-time heli pilot, but a reasonable amount of fixed wing... in lt-mod turb, the kind that makes things a bit uncomfortable in f/w but not unpleasant and absolutely no control issues, the heli (R44 in my case) kind of twizzles around a bit under the rotor. There is no up-and-down movement to speak of. A passenger probably wouldn't notice much. It is much less uncomfortable than f/w. This is what happens for example in the turbulent air coming over the mountains and heading directly for San Carlos airport in the late afternoon.
Yesterday I happened to do a longish flight (by heli standards - KPAO-KLSN, about 70 miles) on a hot afternoon (30-35 C) over dry country, so lots of thermal turbulence. This is the kind of thing that is seriously unpleasant for the pax in f/w, though perfectly safe - if tiring for the pilot. The main movement of the heli was swinging under the rotor. It was certainly noticeable by a pax, but I think much less nausea inducing than in f/w.
I'm a pilot, not an aerodynamicist, so I'm afraid I can't explain exactly why this is.