Not necessarily. Someone might simply be asking for clarification about which person made the soup. "I overheard you talking about someone's soup being really good--whose soup is tasty?" (with emphasis on the "whose")
Or, if there are various kinds of soup all made by different people, you could be asking "Whose soup is tasty? And whose is not?"
Using "tastier" would imply that ALL of the soups are tasty, but some more so than others.