I hate how this gets dismissed as Loki just picking on Thor. He loves his brother and sees how much this whole thing is bothering him so he cracks a joke about it knowing his brother will comment on his own helmet. He wanted Thor to snark back because he wanted him to know he was there for him.
In a land of myth, and a time of magic. the destiny of a great kingdom rests on the shoulders of a young boy. His name, Merlin.
-dragons