I personally feel he sort of is. He does little in any of the movies, even the original, and even less to help the stories along. The only real thing he is implied to have done to help is tell Mufasa where the cubs were...which not only did we not see, but the glare Mufasa gives Zazu afterwards suggests Zazu didn't relly help him at all. In addition to this, it's sort of implied earlier on that he may have been in the graveyard already, since there were hyenas in the pride lands that he needed to make sure stayed out; they could have simply used that as an excuse to have him close by.
Zazu does nothing to help Simba, Kiara, or Timon and Pumbaa. He isn't very likeable, in my opinion, but without even the excuse of being a bad guy. Really, all he does is provide comic relief, which we already have courtesy of Timon, Pumbaa, Rafiiki, and the hyenas.
To quote Ninaroja :"The series is called "The Lion King". Zazu is not a lion, and is only a minor character at best. Like the post above mine says, the sequels are told from the perspectives of characters who have little to do with Zazu, hence his lack of screen time...In any case, the simple answer to this question is that Zazu is hardly in the sequels because he just isn't. There was little to no development of him as a character in the first movie so there was no need for him to be of any importance in anything that followed"
Really, I could not agree more. But what do you think?