in this case ‘C’est vite dit’ can best be translated as ‘easier said than done’
The worried Smurf is reacting to Papa Smurf’s “… et tout schtroumpfera bien” as you suspect, but I interpret Papa’s words to be a reassuring “everything will be all right” (tout sera bien) and not an order to “behave properly.”
With this interpretation of Papa’s words in mind, I read the response at issue as:
“Everything will be all right.” [Yeah, right] that’s easy [for him] to
say [since he’s flying safely away from the danger facing us here at
“Easy” in my English translation above would be an adjective, but another way of translating it (without the specific dig at Papa) would be:
“Everything will be all right.” [Yeah, right] that’s easily said!,
where the “easily” would be an adverb and correspond directly with the use of “vite” as an adverb in the French version.