I wouldn't say so, given it's consistently ranked high on most anime/manga databases, has sold over 50 million copies and is well recognised amongst older titles by most within the manga community.
I'd say it's only really underrated in terms of people talking about the best school comedies, since most would tend to gravitate towards far more generic series such as Assassination Classroom or Shokugeki no Soma due to newer titles warranting more discussion regardless of quality.