They do underrate him, the only people who dont see are the ones who are blind or the ones doing the underrating.
People claiming he missed kaido who didnt even try to dodge, people claiming he is too slow to hit Kaido by himself, people saying the sword has Oden's haki and zoro is not using his own haki to fight, wich is complete and utter BS.
Etc....
You have to be blind to not see people are always downplaying his achievements, another example is how Pica suddenly went from a pretty strong opponent when he almost squished Luffy when he made fun of his laugh to fodder level when Zoro easily beat him, not only that but people also claim he was only able top cut Pica because Orlumbus helped him, even though he sliced a bigger chunk of rock withoud anyone trowing him at Pica moments after slicing the first time.