Underrated in the sense of preconceived notions. Not that Zoro is weaker, but that he *must* be weaker because that's how shonen works. Zoro is the secondary fighter in plot so he must be lesser no matter the context.
This is usually from the same people who complain that Oda no longer emphasizes that styles make fights. Luffy appeared stronger in his second encounter with Lucci and likewise Zoro beat Kaku with strength to spare, but Zoro would've lost to Lucci because he's only number two. Zoro could no sell the jitte but he must be inferior in fighting Smoker, because he's number two. Daz was a natural counter to Luffy's powers but Luffy must still have been able to beat him back then, because he's the number two opponent. Zoro could take his own thriller Bark fights one after the other and then eat all of Luffy's pain and fatigue, but he can't be as tough because he's only number two.
Oda has built in contrivances so that Zoro hasn't actually lost in a fight since obtaining Shusui. That was almost 600 fucking chapters ago. Luffy gets bodied in fights even when he relies on others to help, like how he needed the entire country to get Dofy out of his ass despite the overrated 4th Gear. Luffy literally got fucking walled by a character who was so scared of Zoro she momentarily blacked out without taking damage. For his superior physical strength and the lack of particular pride in it, magically boosted durability and speed, and plain good fortune, Luffy is a less serious fighter. He got hit in the last chapter on purpose, come on. So Zoro has his own advantages. There isn't a single other character we've seen in the present arc with the slightest implication of taking out Kaido with one hit.
People randomly say dumb shit like Kidd is stronger, because he's a captain. I can go on but no, Zoro is not overrated.