I would say that API level tests are mandatory. You want your app to be rock-solid from the start, not crashing on stupid 500 errors you could have easily identified with good tests. Avoiding technical debt from the start is also a really good idea if your app actually kick off very quickly... you will have little time to go back on your first decisions. It doesn't take much time with modern CI tools to setup a proper backend testing infrastructure and totally worth it.
On the other side, i've had some very hard time maintaining a good infrastructure for end-to-end UI tests. These are really subject to change, especially from the start. I would prefer going with solid unit tests on that side.
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I would say that API level tests are mandatory. You want your app to be rock-solid from the start, not crashing on stupid 500 errors you could have easily identified with good tests. Avoiding technical debt from the start is also a really good idea if your app actually kick off very quickly... you will have little time to go back on your first decisions. It doesn't take much time with modern CI tools to setup a proper backend testing infrastructure and totally worth it.
On the other side, i've had some very hard time maintaining a good infrastructure for end-to-end UI tests. These are really subject to change, especially from the start. I would prefer going with solid unit tests on that side.