Is there an out-of-universe reason for the Gundam series mech head designs?
According to Kunio Okawara, the mecha designer for the series:


  "I also put the form of the chonmage, which is a form of Japanese traditional haircut worn by men, and kabuto, which is a type of helmet first used by ancient Japanese warriors, into the design of Gundam. They are my inspirations that are concealed behind the creation of Gundam."