A boxer is supposed to have an underbite. A proper bite was described to me as one that has a space between the upper and lower teeth when the mouth is closed. Some say a 1/4" is about right for an adult.
I'm told that in a puppy you don't want to see such a large space. It was explained to me that usually the space will increse as the puppy grows. No matter, I don't see this as a problem. May create a messier eater and drinker but health wise, it shouldn't be an issue.