Will it look silly if I only cover the top cake ( in the shape of a small shopping bag) with fondant, while the two bottom tiers are iced with bc.
Nope. It is done all the time.
Nope..in fact, it might help the "shopping bag" look more real! LOL!
No- what you are doing is basicly putting a 'shopping bag' on top of a traditional cake- your shopping bag just has the added bonus of being edible