I think the main issue was probably the dryness. I would suggest using milk instead of heavy cream next time, and reducing the powdered sugar slightly. The main reason for that much sugar is to have a stable buttercream base to pipe the ombre and do the drip.

Hi Janet. I found that 2 tablespoons was plenty of heavy cream. The butter needs to whip up very very light. The icing needs to be slightly stiffer to be able to pipe. I live in Florida, where we have a higher humidity, which can sometimes affect the icing. As always, your best judgement should be taken into account for baked goods regarding climate and moisture when you bake. As the recipe suggests, you can also use milk, which is slightly less dense, than the heavy cream and would not have to use as much. 13 tablespoons for that amount of frosting seems like too much. The food coloring also can change the consistency of the frosting, which is why I started with that specified amount of heavy cream. You can also reduce the amount of powdered sugar. The powdered sugar should be added gradually and alternated with the liquid, to ensure a proper consistency. Please let me know if you try it again and how it turns out.

Thank you!