Yes they will. I would let the fondant cut outs dry for a few hours first because the buttercream is moist and will soften the fondant. Smooth out the buttercream as well as you can to make the surface flat for better adhesion. Then just press them on to the buttercream before it crusts.
I've found that drying the fondant isn't necessary. If you have an issue with adhereing it to the buttercream just put a touch of water on the back of the fondant and it will stick.