There are different UI frameworks in development for creating mobile friendly applications.
Kirigami is a nice UI framework from our KDE friends, that should get more attention: it runs natively on Linux (postmarketOS and your average desktop Linux), but the apps can also be built for Android and iOS. Basic testing showed, that the applications work without performance issues, even if there is no GPU acceleration (as it is the case with most devices booting postmarketOS as of now). We might want to move this to a separate article with instructions on setting up a development environment and creating Kirigami apps that work on postmarketOS - help welcome.
libhandy provides GTK+ widgets for mobile phones.
It may be possible to build mobile applications with React Native Desktop, a currently incomplete port of React Native to Qt. It could be necessary to change some things and it might be a good idea to port it to Kirigami and offer additional Kirigami-specific elements just like there are some that are only available on Android or iOS.