When using a customized form in a browser on a mobile device, you will see that the form itself is responsive as to the field layout. However, a sublist element will display just as it does on a desktop, which may require scrolling to view necessary fields. This article describes a way to provide a mobile view of the sublist.
NOTE: The SharePoint mobile app unfortunately does not support customized list forms, even those that have been customized by the built-in SharePoint form customization tools. So to view a customized form, users must navigate to the form via a browser on their mobile device.
Create a mobile view
In SharePoint, go to the list you are using as a sublist, and create a new View that is optimized for a mobile device. For example, perhaps it will only display a limited number of columns.
Use the mobile view in your form
On your customized parent form, add a second List control that contains the Mobile View of the list that you just created.
On the Visible property of the original list control, use this Function expression :
var matchObj = window.matchMedia("(max-width: 767.99px)");
return !(matchObj && matchObj.matches);
The above expression returns true if the window is > 767.99 px. You could adjust this number if you find it doesn't work for your devices.
Then on the Visible property of the 2nd (mobile) list control, use this Function expression (returns true if the window is <= 767.99 px):
var matchObj = window.matchMedia("(max-width: 767.99px)");
return !!(matchObj && matchObj.matches);