There is a major irony in One Piece that's highlighted through Luffy and his crew members. It's the fact that in the world of One Piece, the series' main characters are pirates. Pirates are generally seen as villains who commit heinous crimes, and that's mostly valid. The Marines, on the other hand, are generally viewed as the heroes who fight for justice despite how corrupt they actually are. None of this bothers the main character Luffy though because he has no interest in being regarded as a hero anyway.
Luffy prides himself on being a pirate and that includes how pirates are viewed by the general public. He loves the freedom to do what he wants that the pirate lifestyle allows him. He's selfish in the way he follows no one's rules but his own, and his only rule seems to be to live however he wants. It just so happens that in doing whatever he wants, he constantly ends up being incredibly heroic. Heroes typically help others and save the day because they see it as their duty. Luffy feels no such responsibility but is still a good person at heart.
The Arlong Park and Alabasta arcs are both great examples of how Luffy is heroic without acting in the way traditional heroes do. Both arcs revolve around the homes of Luffy's friends, Nami and Vivi respectively, being in peril due to villainous pirates. Cocoyasi Village where Nami grew up was being ruled by the tyrant Arlong, while Nami herself was forced to work for him in hopes of one day buying back her home's freedom. Arlong, of course, had no intention of letting that happen.
Crocodile, who was a member of the Seven Warlords at the time, was orchestrating a civil war in Vivi's country, Alabasta. He secretly manipulated the people of Alabasta into believing that their King, Vivi's father, was corrupt. As a result, they intended to do whatever it took to overthrow him. This was all so Crocodile could destroy the country and achieve his own goal.
Luffy ended up defeating both Arlong and Crocodile in a heroic fashion. The interesting thing about this, however, is that he didn't do it to save Cocoyasi Village or Alabasta. He did not care about what was happening in either of these places. He simply wanted to help his friends and that's all there was to it. In fact, he had no idea what was happening in Cocoyasi Village or what Nami had been through. What made him take it upon himself to defeat Arlong was Nami tearfully asking for his help.
Luffy's entire journey has seen him saving entire islands from oppression under tyrannic rulers, purely for his friends' sake. Skypiea, Fish-Man Island, Dressrosa, and the ongoing Wano Country arc are just a few more examples of this. Luffy has a unique character trait where he manages to be selfish and selfless simultaneously. This was perfectly highlighted in the Whole Cake Island arc.
Sanji's father, Judge, tried to blackmail Sanji into marrying one of Big Mom's daughters by threatening his friends. Judge wanted this to happen for his own benefit, but little did he know, Big Mom planned to have him and his entire family killed after getting what she wanted out of it. Sanji chose to go along with the wedding to prevent Judge from acting on his threat. Furthermore, he didn't want the Straw Hats to get involved in his troubles. Luffy went to Whole Cake Island without knowing or caring about any of this, with the intention of bringing Sanji back to their crew.
Luffy had no interest in what Sanji's situation was, but he selfishly just wanted his dear crew member back because he couldn't imagine continuing his journey without him. However, he also knew that Sanji wanted to go back with him, despite the latter's best efforts in acting like he didn't. So, he refused to leave Whole Cake Island until Sanji confessed what he really wanted. One of Luffy's greatest qualities is that his friends' freedom is just as important to him as his own. He selflessly refuses to stand by and let his friends stay in situations he knows they don't want to be in or do things they don't want to do.
Luffy's ability to be selfless while being selfish, and save entire countries without even caring about them or intending to do so, is what makes him such an amazing main character. He's a pirate and he doesn't want to be considered as anything else. He doesn't care if his actions are viewed as good or bad, he does what he wants regardless. It's why he can go against the Marines and World Government without caring about the implications. Monkey D. Luffy is not a hero, he's just free.