The ability to automatically sort items by priority on desktop is an useful feature. It keeps them on top in list view reminding that most urgent tasks are to be completed first. Therefore, lack of it on mobile is that much apparent.
iOS version: 11.2.6 (15D100)
Best Answer
As of today, the feature is not available.
You might want to try requesting it through Apple’s feedback site. For more details on how see "How should I submit bug reports and feature requests?" And make sure to read Unsolicited Idea Submission Policy prior to sharing any information.