Well, technically you can’t cover a cake with whipped cream and then put fondant on it. But there are always exceptions to the rules and in this case (cake?!), the Swedish Princess Cake is the exception.
Princess Cake is traditional in Sweden for celebrating a birthday (so I’m told) and in it’s classic iteration it is layers of a white cake, brushed ever so lightly with raspberry jam, filled with pastry cream and then a big dome of whipped cream on the top. (The top of the cake looks like an overturned bowl.) Then the whole thing is covered in a pale green marzipan but with so many nut allergies out there, I gave up using marzipan and started using fondant instead. When I do this, I use a white chocolate mousse or a very firm whipped sweetened cream. It works, but it is best to eat the cake the day it is picked up and when I make this for clients, I put the fondant on the morning they pick it up. Usually I put the cake in the freezer for a few hours as a little insurance to make sure the dome is firm enough. And it’s the dome that helps because there are no sharp edges on this cake which means you don t have to smooth the fondant (or marzipan) at all if you do it right. I tell clients that it will hold for the day, but no guarantees on what it looks like the next day It is a huge seller for me in the spring, especially for Easter.
As for using fondant to cover a cake slathered with whipped cream (sides, top, filling…) I don’t think it’s a good idea; you can’t really smooth the fondant out the way you do on a typical cake. Maybe you can use fondant dots or some other small design made from fondant on the cake?