"Yaroo" on the end of bakayaro makes the "baka" part a bit more vulgar. It is like calling someone a moron, idiot, ***hole, if you get the drift. The familiar form, and the congregational tense implies that the person referenced is willfully being a jackass and making a spectacle of himself. The vulgarity is highlighted by verbally calling attention to the person for his actions, and potentially bringing him a loss of face due to your implication. It is a common way to put someone down or 'dis' them for their actions.