I don’t think PINE64’s decision to preinstall Plasma Mobile was based on the technical merits. Both postmarketOS and Manjaro chose Phosh as the best interface to represent their distros when they released PinePhone Community Editions, despite having Plasma Mobile as an option and working on a Lomiri port. If you follow the comments on the PINE64 forum, both Phosh and Lomiri are more usable interfaces at this point in time.
Instead, Plasma Mobile fits PINE64’s model of community-based development, and Plasma Mobile has adopted the PinePhone as their target device for their development work. By preinstalling Plasma Mobile, the PinePhone probably will be able to help stimulate the development of Plasma Mobile, so it isn’t a bad decision from that point of view.