According to Siliconera's translation, the visual concept for the jersey relates to Adidas Japan's Enjin (Ring) Project, the name of which references the circle made by players huddling together. If you can imagine a team wearing this, hunched over with arms above each other's backs ... yup, the red stroke across the shoulderblades would form a ring. It's almost like somebody planned this or something!
If you're still lost on how Pikachu relates to soccer in the first place, Siliconera adds that the choice is meant to get more Japanese fans invested in their World Cup team, with an emphasis on drawing in younger fans. That makes sense - what appeals to younger kids more than Pikachu? More than candy and recess, anyway.