No matter if you make a 95% OO soap and 5% castor, the castor will help to boost the little bubbles produced by the OO soap. In fact, in any recipe when you start to get to higher amounts (think 15% and over) castor, you start to reduce your bubbles.
So - only with coconut? No, castor will improve the lather of any soap provided the total castor amount is not high. But castor itself does not make bubbles.
Experiment for yourself as well. I made one recipe with lard, sweet almond oil and coconut, but no castor oil (cause I was out) and everyone commented on the really nice lather. Carolyn has found that decreasing the castor really slows down her trace. I'm not dissing the castor oil, but I'm not going to argue that its critical either.