You will have to ask each time. Typically, a smoothie is not considered a standard fountain drink, but some places might allow it. I believe the dining venues at Coronado are operated by a third party so in that case most likely it will not be allowed since they are reimbursed by Disney according to the rules of the plan.
On allears and The Mouse for Less there is no purple ddp logo next to any of the smoothies so it does not qualify as a snack credit item.