I use Flutter for my day job, which I highly prefer to native iOS and Android development.
(If I ever have the time and energy, I am considering rewriting MobMuPlat in Flutter, which would unite 4 separate implementations (iOS, Android, Java Swing for desktop, MacOs) into a single implementation.)
While a Flutter GUI (and other layers of the app, e.g. data layer, controllers, etc) is cross-platform, you will still need to have a 'native' layer for iOS and Android in order to handle libpd integration and its interaction with per-platform audio APIs. But that would operate the same as existing iOS and Android native apps which use libpd; you would merely add a 'plugin' wrapper (i.e. a Dart library which translates app logic into calls to the native layer).