I took a look and don't see any strong reasons not to use and support the official app. Tweetbot was so successful because it was competing against the official client which was (and still is) truly horrible, but I don't see that in this case Ivory has many advantages over the official client.

I used to use the official client prior to Ivory. It's a fine app, but it's a far cry from Ivory's polish and overall quality — I mean, it feels snappier and more “at home” on iOS than Mastodon's official app.

Anyway, Mastodon has no reason to stand against third party apps. Its official app was released only last year, and by no means to threat others nor become the only app in town. Experimentation is good, and Mastodon's third apps are shining right now. There are dozens of them, each one bringing fresh ideas and new concepts.

edit: typos.

The official app makes it awkward to read the local timeline, which is where the real fun is on smaller instances.