I've tended to notice normal seeing deteriorates as clouds approach the field of view, then improves back to normal as they leave the area. Not necessarily some of the best seeing between clouds, just better than in the clouds. To my way of thinking, it makes some sense. Aircraft usually hit some turbulent air passing through cumulus clouds.
Different kinds of clouds form for different reasons. Cumulus clouds form at the top of thermal uplifts, almost always during the daytime. It's that thermal uplift that causes turbulence. Stepping back from the details, the patchy, clumpy nature of cumulus clouds implies some kind of inhomogeneity, and inhomogeneity is what causes bad seeing.
A priori, one would expect clouds that form in uniform sheets that stretch over large areas of land -- such as altocumulus -- to be a sign of homogeneous atmospheric conditions. So it would make sense for them to be associated with good seeing. Those are, in fact, the kinds of clouds where I have experienced good seeing.
But over-simplified science like what I've done above, together with snippets of anecdotal data, are not a substitute for systematic data.