There are many important types of testing that should be done on an iOS app to make sure its functionality as well as quality. According to me, here are some most important testing that you should know:

Functional testing: This type of testing makes sure that the app is good in every function as it needs to and that all the features are working as expected.
Usability testing: It is the type of testing that aims on the user experience and makes sure that the app is easier to use, intuitive and satisfies every need of its target audience.
Performance testing: It analyses the app’s performance when it comes to speed, stability, response time and satisfies every need of its target audience. Platforms like HeadSpin can help you perform such type of testing easily.
Regression testing: In this type of testing, you can make sure that the changes or updates to the app have not damaged its functionality or features.

So, these are some types of testing that should be performed on an iOS app.