Wearing a helmet with horns is a really bad idea in any battle involving swords or axes. A helmet is designed to cause blows to glance off the head, deflecting them to a course tangental to the curve of the skull. Sticking a great big horn out the side of the helmet has just the opposite effect: it can deflect the blow to a course more perpendicular to the skull, thus increasing the likelihood of damage.
The Vikings were almost certainly too smart to go into battle with this sort of death-trap stuck to their heads.