Can I use whipped cream icing underneath the marshmallow fondant?
I only use MMF and I wouldn't suggest using just whipped cream under it since it's so heavy and sensitive to moisture. I would probably suggest using whipped cream as maybe just a filling in between the layers of cake (with a buttercream piped dam around it) and then using some more sturdy buttercream to coat the whole cake. I've never tried just whipped cream under fondant though so if you do, please let us know how it goes :) Happy caking!
Salt Cake City
I agree with SaltCakeCity, Fondant is so sensitive to moisture. Whipped cream is best used only between the layers. The fat in the buttercream acts as a barrier - therefore keeps the moisture from the cream in. I would place the cake in the fridge to chill for an hour or so after filling it, and again whenever you coat with buttercream. - to FIRM up. Chilling makes the cake more stable and much easier to cover with fondant.
Hope this is of some help!