People are becoming more aware of fondant-and its taste-so I think they look at a cake like your's and think, "Yuck!" so you have to tell them it's made of marshmallows and not powdered sugar and water. You may want to take your leftover bits and break a piece off for people to taste so they can spread the word that it tastes good. A lot of decorators end up peeling off regular fondant before serving because it tastes bad and no one will eat it, but to me it seems like such a waste of your time and all the pretty to do that. I can understand why they do it, though. Who wants to see a bunch of sour faces when they taste your creation?
I got the Marshmallow Fondant recipe from the Wilton site. It's tough to get it going. I have to use quite a bit of shortening on my hands to get it kneaded good enough to roll and cut; but you don't taste the shortening in the end product.
I do make my own buttercream from the Wilton recipe. I like the taste and texture better than canned frosting. I have a really weird tip for you that no one ever believes works. Once you have the buttercream on the cake, put the cake in the fridge for 5-10 minutes, until it crusts over a little. Take a NEW (as in, has never touched paint) small foam paint roller and handle and roll it over the cake to smooth the frosting. Don't be afraid to apply a bit of pressure to get it to smooth. If it's too sticky, put in back in the fridge. If it's still too sticky, the buttercream probably isn't stiff enough so you can try adding a sprinkling of powdered sugar over it but that may not work as well on colored frostings (just pure white buttercream). I tried the putty knife trick, the Viva paper towel trick, and everything else out there but I couldn't get it to be as smooth as bakery cakes until I grabbed a foam roller in frustration one day.
If you want pure white buttercream, make sure to pick up a bottle of clear Vanilla and a bottle of butter flavoring, and substitute the butter in the recipe for more shortening (NOT butter flavored shortening). The Crisco brand shortening is a bit tough to use right now since they took out the trans-fats, so look for a generic brand that still has transfats in it.
I also can't cut a cake straight for nothing so a couple of minutes after it comes out of the oven, I press a potholders on the top to flatten it. Don't be afraid to apply some pressure with this tip, too. "Real" cake decorators will tell you that hate it when people do this as they feel it makes the cake too dense, but everyone loves how my cakes taste and the texture so if it's the only flat to get the darn thing even enough so it doesn't hump, I'll do it.
I could talk about cakes all day!