Did former STAR C Shaquille "Shaq" O'Neal underachieve in his NBA career ?

No, he was a successful NBA player who made the Hall of Fame.

Hall of Fame, four titles, 15 ASG appearances...   pretty good resume, if you ask me.  Some people argue that if he had kept himself in peak physical condition his entire career, he could have accomplished even more.  But it's also possible that the stress would have caused his body to break down much sooner.  Shaq is  a unique physical specimen, it's difficult to tell.