Most of the time his head isn't necessarily like a box, though. The front of his head is squared out, but at the back, especially at the crown of his head it tends to round out more. I totally agree that purely square heads look ridiculous on Bombermen, but those usually occur in fanart that misinterprets the shape of his head. So it's more of a question of whether or not they look better with round or square faces.

There are characters who need round or "regular" heads, but others look better with squared faces. For example, Great Bomber would look weird with a square face, but Hammer Bomber's square face seems only natural (try to imagine him with a round face... even the visor would become distorted). In my own opinion Shiro looks best with a square face, since that's how he always was before... I have nothing wrong with the Jetters style though.