The two kind of feed into each other. The UI framework is conducive to polished UI, which results in more polished apps, which raises the bar of expectations for users. This then pressures developers to deliver more polished apps.

This is why I think that anybody looking to create a new platform with a similar culture of quality indie apps should put building an capable, pleasant native UI toolkit at the top of their list of priorities. UI toolkits are the lifeblood of platforms.