I made some cupcakes for a class party. Baked and iced them yesterday and sat a fondant flower on top. This morning, I could tell that the flowers were already looking "gooey"/"melted" (for lack of a better description) looking.

I've had this to happen to cakes, in the past. The fondant that I applied to the cake would get this way after a day, or two. The shape stays the same, but it just gets gooey.

Does the buttercream break down the fondant? Is this normal?

I use Wilton's buttercream recipe.

Buttercream has humidity. Humidity is the archenemy of fondant. The only solution is to put the fondant accents on as close to the time of the cutting of the cake as you can, unfortunately. It would probably help to use a crusting buttercream, as well, and let it get as hard as possible on the surface before putting the flowers on. But I think Wilton buttercream DOES crust, doesn't it?