You can pretty much always count on a hero to:

• Leave their ordinary world to pursue a mission in unfamiliar circumstances.
• Discover new friends and make new enemies, including one primary foe who seems undefeatable.
• Face tremendous trials.
• Learn difficult lessons.
• Experience an “all is lost” moment or a moral dilemma. Or both.

And

In terms of what the hero is like as a person, traits can vary. However, the most common qualities seen in the hero archetype are:

• Physical or magical strength.
• Physical, mental, and emotional resilience.
• Persistence.
• Courage.
• A strong sense of right and wrong.
• Commitment to a mission, relationship, or value system.
• An impulse to protect the defenseless or give a voice to the unheard.

This dude is a 1:1 match for the hero archetype lol. Name one example where Luffy decided not to help or choose a mutually beneficial solution to another character that he liked. Helping only friends and kind doesn't make you any less of a hero.

There's nothing wrong with following the archetype, it's actually a sign of good story writing.

But having him say "I'm not a hero" every five seconds is annoying as hell and feels like shoehorned dialogue that doesn't go anywhere or build on anything.