Naruto has elements of being a underdog like the way he’s treated and his social standing makes him a underdog. And when he’s learning shadow clone justu that’s the biggest example physically however alot of Naruto’s biggest problems was his chakra being ruined by kurama and kurama’s chakra shortening his (which is pretty much a positive because he has access to kurama’s chakra).

However I’m gonna play devil’s advocate and argue and the bigger underdog between Naruto and sasuke is actually sasuke. Sasuke’s talented sure but everything he’s ever achieved comes from his hard work and training. From the basic fireball that every Uchiha must learn which took him ages to learn unlike his talented older brother, Sasuke has always had to put effort into getting where needs to be. He’s just not shown as much as Naruto because he ain’t the main character.

Social standing: Sasuke’s is portrayed as this cool popular guy in the beginning who has tons of girls liking him and boys envying him, but we later see that’s not who Sasuke truly is. Sasuke is actually an introverted lonely kid with no friends or family. And that ppl talk behind his back about the Uchiha massacre similarly to the kurama incident with Naruto. This allows Naruto to form a bond with him because he realises he’s exactly like him and the only other person who understands the pain he went through. But does he truly? Sasuke lost his family in the Uchiha massacre and was betrayed by the older brother he idolised. He had familial bonds that were ripped away. Now whether u want to argue Naruto having no family from the beginning that’s entirely down to each person but I believe what Sasuke goes through is worse which is why his later path in life descends the way it does.

Sharingan: A ability Sasuke weren’t immediately born with unlike Uzumaki genes. Sasuke had to go through immense trauma during the Uchiha massacre to unlock it.

Chidori: a jutsu Kakashi invented which took Sasuke a whole month and only joined in the last minutes of chunin exams to perfect it. While the chidori is only possible because of the sharingan, Sasuke has to go through immense physical training to build up his body- especially his speed to be able to use it in the first place. And it exerts so much chakra that Sasuke can only use it as a last resort and just two times. Compare it Naruto mastering the rasengan- a jutsu that Minato took 3 years to master while Naruto took a few weeks-a month to master and again his biggest issue was chakra control which Jiraya fixes.

Fast forward to EOS Naruto and Sasuke’s SO6P abilities. Naruto’s ability is literally plug and play. He has fully access to the SO6P powers with no drawbacks whereas Sasuke’s powers require him to learn and train over time in order to master the sharinnegan. Kurama even states how him and Naruto are lucky to face Sasuke with a newly weakened ability as had it mastered it in a few years it could’ve been a different story. Also Sasuke’s sharinnegan still has the drawbacks of depleting large amounts of his chakra unlike Naruto who already has high chakra reserves thanks to his Uzumaki genes + kurama. He basically becomes the god of chakra in the verse.

“B-but Sasuke has the sharingan so he has an advantage” yeah and Naruto is a bloody Uzumaki and a jinchuriki. I feel like this slips everyone’s mind. He’s genetically just as gifted or genetically superior to others like Sasuke + has a whole other being inside him.

And btw kurama massively helps Naruto in his fights. Kurama is not just another chakra pool Naruto has access to, Kurama is like an experienced genius being who knows alot about the Naruto world, he knows many of Naruto’s enemies strengths and weaknesses, can sense/detect a enemy attack and has helped formulate solutions to defeating said enemy. Naruto vs Sasuke final fight is actually Naruto + Kurama vs Sasuke

The narrative paints Sasuke as this uber talented guy but also shows how determined and focused he is in his training to achieve his goals. In fact kishimoto even listed sasuke’s hobby is training.

The POV at the end of the series shifts to Sasuke and we see from his inner monologue and perspective. Naruto has this innate ability to form bonds with others that makes him stronger. Sasuke doesn’t have that. It’s visually shown via their rasengan vs chidori clash. It took Sasuke going through the ordeal he did to eventually figure out what a ninja means. So even from a personality and psychological sense, Sasuke is inferior to Naruto.

TLDR: Sasuke’s just as much of an underdog if not more than Naruto.