One Piece Episode 520

[DarthEnderX]

I'm not sure if Luffy is actually looking forward to a big fight or if he's traditionally just too stupid to see one coming.

I think he's specifically trying to avoid a fight. Hence the mustache.

I'm a bit confused, wasn't the hat that chopper wore a precious gift from Dr. Hilruk. What happened to it?

Good question. Maybe Oda realized that having two characters with keepsake hats was repetitive.