Some interesting interpretations on the evolution of his helmet over the past 40-odd years.
You do realize that he has always had the same helmet and different artists simply provide their own interpretation, right?
He had a “G” in the middle of his chestplate.
WHY?
He’d say it’s so he wouldn’t grab some other intergalactic destroyer’s chest plate by accident, but actually it’s pure vanity, all the cosmological threats have monogrammed armor and he didn’t want to feel left out.
Only to humans. That’s the way we perceive him. Sorta like a Vorlon.
I thought he was a cloud? (Forgive me… 
)
Given the fact that his name means “milk”, I guess it’s not too surprising that he’d go for cutesy-poo headgear.
That’s actually his family crest from back on Taa.
Um yea like I’m buying that, next you’ll tell me Superman’s “S” is a family crest in Kryptonian too. 
Why did Galactus have a “G” on his chest? You might just as soon ask why all the Marvel giant lizard monsters wore underpants.
Here’s his true form (on the left).
I want to carve my own Galctus head tiki mug with the reverse flying buttresses on his head as the handles.