Cell arrays are practical if you need to store elements that have inconsistent format or dimensions, but for what you are trying to do, a 3D array is easier to work with. Here is what I would do:
Preassign a 3D array:
coefs = zeros(128, size(J, 2), size(J,1));
then compute and populate the stack:
for ii = 1:size(J, 1)
K=J(ii,:);
coefs(:,:,ii) = cwt(K,1:128,'morl');
end
Finally, compute the mean along the third dimension:
MeanCoeff=mean(coefs, 3);