It's not. There's Goemon's Great Adventure, Tarzan, Mischief Makers if you count what little 3D it uses, probably more.
I'm really surprised to see so much Kirby 64 hate. The game was overall very well-received and still is held in high regard. It's a little less well-known compared to other N64 platformers, especially for one published by Nintendo, but I'd chalk that up to how late of a release it was. Honestly, I'd rather play Kirby 64 than many of the bigger name platformers on N64. Like I just recently played Banjo-Kazooie for the first time (my first time with Kirby 64 was about two years ago, so I'm not going off of nostalgia before anyone suggests), and I definitely prefer Kirby 64. It's a bit too easy (but that's nothing unusual for a Kirby game) and it's a bit slower than some other Kirby titles (but it's comparable, if not faster, than many, many other platformers that are highly respected, like much of the Castlevania series), but other than those issues, I really can't come up with many complaints about it.
From what I've seen of it in action, Dream Land 3 looks like a really nice game too, but I can easily see how if someone disliked one, they'd dislike the other, considering that Kirby 64 is basically a re-envisioning of the game.