Katsuki was being himself, grumpy. It's early in the morning and he just woke up and is eating his breakfast before school can start. That's when you came out and went to the kitchen. You hair was tied up and it was messy. It's obvious you fell asleep with you're hair tied up.

"Happened to you're hair, dumbass?" Katsuki asks, as if teasing you. Yes he is teasing you.