Yes Sugar, but that is what I meant, with these cakes you need a stiff filling and whipping cream doesn't give you a stiff filling as it never really firms up enough unless it is used in a small amount, just as the liquid added. Oh you likely mean, because I referred to it as an icing? Sorry, meant to say filling, but actually I see no reason why, on an ordinary cake, this could not be used as an icing, I have seen it used in bakeries.
Yes, if you are using something in a large quantity as a filling, most particularly in a case when you are using it to build up height, which is really what you would be doing in this case, to get the slant, then you need something really stiff. So whipped cream fillings such as this or mousse and those types, even puddings, are not the most stable of fillings when there is going to be a lot of weight on top like in the case of a stacked cake or a whimsical cake. Worse if your cake is covered with fondant.
If you use a regular buttercream icing, normally you add 2 tbsp. as liquid, well then you can use the whipping cream as the liquid. So in a regular recipe with 4 or 5 cups icing sugar, this amount of liquid would give you stiff buttercream which normally you would thin down for purposes other than the stiff buttercream dam. So with only the 2 tbsp. added (I should use the Wilton recipe with the 1/2 butter, half cream as an example, this uses 2 tbsp. of liquid) that would likely be a good stiff icing to use if you were going to try slanting your cakes strictly using the icing to get the slant, not cutting the cakes to get the slant. Of course you would have to insure that the icing was a bit sticky so that it would hold the cake in place. If it is too dry, it won't adhere the cake to itself and you may get slipping issues.