If something is soppier, it has more liquid in it, especially if it's too much liquid.
Soppier can make people think of wetness, messiness, or too much of something.
Examples:
The sponge is soppier than the towel.
The ground is soppier after the rain.
She made the bread soppier by adding more water.
The dog's nose is soppier than mine!
The ocean is soppier than a lake.
The soup is soppier than the stew.