Stubborn is ok. The problem is the plot convenient.
Once Luffy get beaten, he can still be stubborn but he should be more careful, aware with the consequence, or at least try to come up with strategy, like the time he fight Crocodile.
Sadly post timeskip plot works in the opposite way. He is super carefree and he doesnt need to try. Its like he knows the plot gonna feed him perfect teacher, power ups, or flashback. This makes his story too unrealistic and unrelated, and make Luffy become a shit character.