Your y-labels aren't really tick labels. They are text objects. You need to store their handles and set their fontsize directly.
textLabels = gobjects(size(ulab));
for k = 1:numel(ulab)
textLabels(k) = text(x,loc(ind),lab(ind),'Color',clr(k,:));
Then change fontsize
However, the whole approach seems inefficient.
Instead of defining the labels at the top of your code and then change the labels within the loop, just set the labels correctly before making the plot. That way your lables are actual y-tick labels and will respond to set(gca,'fontsize',18).