It’s no question that names matter among the pirates in One Piece and Oda has emphasized it with Luffy’s unique way of introducing himself to everyone he meets so when Oda chooses to dispense of Luffy’s actual name, there’s usually something interesting tied to why he does it. I think whenever Oda makes Luffy take up an alias, Luffy intern actually changes character slightly and for a good reason.
The three times this has happened significantly(enough to warrant an Oda Box) are Lucy the Gladiator, Luffy-no-Umi the Sumo Aspirant and Luffytaro the Samurai. Granted there’s also Afro Luffy and Nightmare Luffy but then those weren’t personas taken on to fool the people around him so people still know it’s Luffy.
The first two, Lucy and Luffy-no-Umi are aliases used for competitions which as a concept is very diametrically opposed to what pirates are. As seen in the Crocodile and Katakuri fights, there’s no such thing as sportsmanship in the pirate life. And yet, as Luffy-no-umi, Luffy does very strictly adhere to the Sumo rules. He struggles to stay in the ring, only uses open palm strikes and even calls out his opponent for cheating and yet he never said anything during the Flampe incident in the Katakuri fight. Another thing that competitions entail is showmanship, which can be a pirate thing like with Buggy and all but it’s definitely not a Luffy thing. Lucy on the other hand was actively working the crowds during his tournament. This is the same Luffy who couldn’t care less about Bellamy giving him hell at Jaya in front of people but at the Colloseum he was eating up the attention as Lucy.
Last but not least though is Luffytaro, the man Luffy himself said will turn Wano into a paradise again, in true Hero fashion.
But we also know that the last time this issue of heroes was brought up, Luffy ended up fighting Jinbei. According to Jinbei, Luffy should’ve attacked Hody at the last possible second, again in true Hero fashion so that Luffy becomes a hero to the Fishmen as opposed to some human pirate who just wanted to beat up a Fishman pirate because humans are just dicks. Of course Jinbei managed to stall Luffy long enough to bring about this outcome but Luffy did it begrudgingly at best and we all remember Luffy’s words about meat and heroes. Then comes Luffytaro recently saying he wants to actually leave Wano as a paradise even when he has no actual stake in it once he leaves. This isn’t Luffy wanting all the meat for himself because he won’t be there afterwards. This is him wanting the people to have all the meat once he fights to provide it for them. This is Luffytaro, a Hero.
I suppose you could call these aliases cheap ploys for Oda to get Luffy into certain situations and have him react in certain ways contrary to how he usually is but I think it’s the opposite. Luffy himself said he like heroes, but just didn’t want to be them. This tactic of using aliases to augment his personality is Oda being the most clever when characterizing Luffy.
Essentially Luffy is in fact a good kid, but he is also not naive at all about the life he chose. He knows the stigma around pirates and knows what is required to be a pirate. But as a good kid, he would want his good actions to be rewarded with applause like at the colloseum. I think he’d prefer fights were more like sports, where rules could apply and winning was based on merit. He is also a likable guy so why wouldn’t he become like an entertainer and work crowds and live off his natural traits other than having to work so hard and fight countless enemies. He also likes helping people and with the right amount of strength, people who help people should be considered heroes. All these things though are almost impossible with the mantle of a pirate and Luffy fully accepts it.
Consequentially though Oda also gives Luffy ways to live vicariously through these aliases, paths he can’t really go down now. That’s where the genius of these aliases comes in, they show what Luffy would be like if he lived in the world where he was applauded for his action, was helping people to just be a hero to them, and lived without the turmoil of almost dying by random things out of his control but, Luffy always chooses the pirate life every time after living as these aliases and so reinforces his belief in his way of life as opposed to just settling for those other paths.
Conclusion: There’s that common discussion about why pirates don’t just sail without Jolly Rogers in order to reach Raftel and not get bothered by Marines or hated by civilians. It’s just because you won’t be a pirate anymore, because pirating is a way of Life in One Piece not just a means to an end(like in our world). Luffy takes up different names at times because situations don’t always need, Luffy, the man who will be king of the pirates, and he is smart enough to recognize that but also wants to see what it’s like to not be the man who will be king of the pirates for a time. This isn’t presented as an internal conflict though by Oda, it’s instead used to reinforce why the man who will be King of Pirates is more important because Luffy always leaves those aliases behind nonetheless. Similar to sailing without a Jolly Roger, Luffy could decide to live as a hero(marine or not) and sail to Raftel trying to amass only good will but that will change his core concept of living free as a pirate by shackling him to strict moral courses of action like NOT destroying Impel down just to save one man. In this way, he would want the goal of being a pirate without living the pirate life and we know from Sabaody, Luffy wants the pirate life and adventure way more than just ends.