I think there is an element of truth to this as after 5,000+hrs gliding I sometimes seem to be able to detect the presence/whereabouts of a thermal by associated turbulence; can’t really tell you how I do it
. The roughest days are often those with lower lapse rates (more stable) as the thermals tend to be narrower with more defined shears on the edges, as entrained air does not have natural buoyancy. Same thing happens with sinking air.
I wouldn’t say that much of the turbulence associated with convective conditions is due to the above theory as it can be pretty bad on days with no vertical shear (or wind).