why do spartan helmets have a mohawk of hair on the top

That crest, or the 'mohawk' as you call it, wasn't just for looks back then. From what I've picked up over the years, it served a couple of practical purposes in the middle of a chaotic battle. First off, it made the soldiers look a whole lot taller and more intimidating to the enemy, which is a pretty solid psychological trick when you're staring down a line of guys trying to take your head off. It really helped stand out in the mess of a shield wall too, so you could spot your commanders or fellow unit members easier.

There is also the matter of protection. While it looks like just hair, it actually helped deflect downward blows from swords or axes away from the top of the skull. It acted a bit like a buffer zone. I've always thought it was a clever bit of engineering for the time, even if it looks a bit wild to us now. It’s funny how they managed to combine something that looked pretty fierce with actual functional gear.