Hunter X Hunter is about Gon and Killua.
Digimon is about Taichi and Yamato.
Death Note is about Yagami and L.
Naruto is about Naruto and Sasuke.
Doraemon is about Doraemon and Nobita.
4 of the 5 thing above is as legendary as One Piece, which means...
One Piece is about Luffy and Zoro.
Obviously tho, Zoro isn't built like those guys. Those guys are blatantly the MC counterpart.
With Zoro, it's more "under the radar". He could've been the MC counterpart like those guys, but he isn't.
But if we look at the subtle writing and parallels, it becomes quite clear that Zoro's role, while not exactly Sasuke/Kilua/Vegeta, is similar in more ways than his haters are willing to admit.
- Zoro is the only character other than Luffy to have a global ambition. Haters always point to "but Zoro abandoned his dream in Thriller Bark". No he didn't, it's more of Zoro doesn't want to become WSS if that means Luffy dies. Luffy said pretty much the same thing when it came to Ace - that he wouldn't want to become PK if he can't go save Ace, the regret would be too big. And Luffy did almost completely break down and give up on his dream, when Ace died, Jinbe had to talk him out of it.
- mid gen parallels with Shanks and Mihawk, two characters who are the great rivals of their generation similar to Roger/Whitebeard before them
- Luffy and Zoro are the only two characters having portrayal with those referred to as "Gods" of centuries ago. Joyboy - the sun God, Ryuma - the God of blades.
And in Wano, what "Joyboy" Luffy did was directly compared to the legends on Ryuma. They were also the two characters contrasted with Oden.
- direct contrast with Sun God Nika/Demon God Asura
Oda's writing with Zoro is more subtle than the MC counterparts in other main shounen anime like Vegeta or Sasuke. But that is by design, Zoro is meant to go under the radar.
@nik87 put it the best, with Zoro it is
"show, don't tell"
But yes, he certainly isn't as important to plot as say Sasuke is.