I did a test, using cumulonimbus Floyd Epic Clouds preset, default clouds density is 0.08. Flying through the clouds with the Pilatus that is a light plane, there are no turbulence. Setting density to 5 (max), same, no turbulence (clouds look like volcanic eruption). So my guess (same as in other posts) is that the turbulence we can sometime experience flying through clouds are coming from the ground or from gusts injected by Live weather but are not generated by the clouds themselves.
In SU6 they made an adjustment to the cloud density effect on the aircraft but it was to fix a performance problem with the aircraft, it did not impact the turbulence within the clouds.
Continuing the discussion from [RELEASE] Release Notes (1.20.6.0) Sim Update 6:
Maybe they’ll make some adjustments to the density in the next SU15, it will improve clouds depiction that in many cases are too transparent in Live weather for sure but I doubt that it will fix the turbulence issue within clouds. They should find a way to link the cloud density (or another parameter) to a turbulence value or correctly model the up/down draft flows, air flow turbulence generated within the clouds.