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Greninja doesn't have foam and uses its tongue as a scarf because frogs covered in foam are associated with their young.
The foam is used to create nests, and thus, since Greninja is fully evolved and essentially the "adult" of the line, it loses the coat of bubbles around its body to signify its maturity.
However, because it's a NINJA and because scarves are an important visual element for such characters, Greninja takes a second option and utilizes its overly long tongue as a scarf.
There is no "less thought" put into the design. They did not abandon the theme.
