What's the purpose of the decoration on the BrickHeadz piece?
According to LEGO, it's the BrickHeadz icon with the series number.


  He also stands on a buildable baseplate with series number and BrickHeadz icon for easy display -- Batman Brickheadz LEGO Shop page


If they release another series of Brickheadz, those sets will probably have a similar piece with a "2" on it.