Can an app that runs on iPad be forced to be displayed in iPhone's resolution or aspect ratio? The example is Pokemon Go: when it is running on the iPad Pro, almost everything is fine, except you cannot see part of the upper screen and part of the lower screen.

As a result, in a gym battle, when your move is super effective or not very effective, the message at the top of the screen cannot be seen. And when you try to catch a Raid boss, the number of Premier balls remaining at the bottom of the screen cannot be seen either. (unlike iPhone).

So can you force the app to run on the iPad Pro like it is an iPhone app (or use the iPhone's screen aspect ratio -- long screen size instead of 4:3)?

(The app is published as an iPhone / iPad universal app, and all it takes is one click for the developer to publish it as an iPhone only app, and it will be running as an iPhone app on the iPad and everything will be visible on screen. The fact that the developer chose to publish it not as iPhone only app but with some elements not visible on screen on the iPad, is really a bad decision and therefore the user really should be allowed to choose to use it as an iPhone app on the iPad).
No you cannot

When the developer creates the app he/she specified the screen aspect ratio in the development software. There is no way to change the aspect ration to fit your iPad Pro.

You could recommended to the developer company to create a iPad Pro version of the app that had the proper aspect ratio for your iPad Pro.