So I was lurking a thread and it turns out the thing about baby frogs and bubbles isn't true.
Many people said it was logical for Froakie to lose the bubbles because that's what happens to real frogs, but that's actually a myth, and that Greninja can't have a bubble scarf because its illogical and "doesn't happn in the real world"

There is no species of frogs that licks away its bubbles when it matures. One species of frog, the Tungara Frog, lays the eggs inside of bubble-like foam, Soon the outside of the bubble nest dries and hardens into a shell. That protects the eggs and keeps them moist. When the tadpoles begin hatching from the eggs, the nest begins to fall apart, they then burrow through the foam and fall into the water, without the foam bubbles.
http://www.youtube.com/watch?v=azJGGd9FFnE
https://www.nwf.org/Kids/Ranger-Rick/Animals/Amphibians-and-Reptiles/Frogs.aspx

What's unique about the Tungara is that most of the other frogs lay their eggs in jelly-like masses.

The argument about Greninja being a frog that "licks off" its bubbles as it grows older is not true. So its perfectly valid that Greninja could have had a bubble scarf, but for some reason didn't.

Does Greninja always have its tongue sticking out? maybe he still blows bubbles, it's a possibility that he has both.