Jenrya didn't come up with his head ornament. Terriermon didn't either. One of his friends from school had a boyfriend with a suspiciously real looking Terriermon, and he always wore it on his head.
The future Tamer asked about it and Wallace only grinned, "He's a great hat. Perfect for shade, if only a bit heavy."
Jenrya didn't understand the weight part until his Terriermon climbed onto his head to peer at something in the distance.