Zoro is not the deuteragonist of one piece
I know it sounds crazy and that's what fans thinks and it might not even be wrong. but fans believing one thing and the truth being entirely different is not really a new thing. It happens most of the time.
From what google says :
deuteragonist
/ˌdjuːtəˈraɡənɪst/
noun
the person second in importance to the protagonist in a drama.
deuteragonist is someone who has the most importance in the story after the mc and zoro doesn't really fill that criteria. We can take Vegeta and sasuke as bases for this because we know for certain that they are the deuteragonist in there respective shows. without them the story wouldn't advance or would be entirely different. We don't really need to talk about Vegeta on that matter it's obvious as day. for sasuke he is quite literally half the plot of shippuden so also self explanatory.
For zoro however that's not the case. His role in series is being right hand man and defeat the second in command or the second strongest of an arc (mostly) but even without him story can advance. Luffy or sanji or probably jimbei would just have one extra threat to deal with without zoro in the story. the show would be a lot different without him but it wouldn't really change it that much.
then the question would be.. Who is the deuteragonist of one piece??
Answer to that would be pretty complicated... or maybe it's just simple like one piece probably just doesn't have deuteragonist and all the straw hats are just main side cast including zoro.
But since that's boring.. we can technically make an answer. By the definition, the deuteragonist would probably either be nami or robin. If you are a op fan you would know why i am saying it's one of those two. It's because they are the most important after luffy. Because without them he can't reach the one piece. and the story wouldn't advance. It's proably one of these two. But if we are going with the traditional route of deuteragonist appearing right after or earlier than the mc in the story then it's Nami. which makes a lot more sense due to her dynamics with luffy.
Great! so now you know what to say when someone says or ask if zoro is the deuteragonist of the story.
Your welcome.