In general, Cesium itself can work completely offline as part of a mobile app. You just need to point it to imagery/terrain/data that is also part of the app. Plenty of devs on this forum have had great success with this strategy on any number of platforms, iOS, Android, etc…
Flutter itself does not support WebGL directly, so in order to use Cesium in a Flutter app you would need to embed it in a WebView. I quick search shows that people have had mixed success with this approach, for example this comment here: https://github.com/flutter/flutter/issues/29892#issuecomment-541781217. So it might work great for you, depending on your exact use case and target hardware.
Once inside the WebView, there’s nothing special about Cesium regarding how you interface with it, it’s just like embedding any other web content.
Hope that helps.