以下是您需要的提示：

return GetMaterialApp(
  theme: ThemeData(
    textTheme: TextTheme(
      // Setting fontFamily for the bodyText1 text style, that is used by default for the Text widget.
      bodyText1: TextStyle(
        fontFamily: 'YourAwesomeFontFamily',
      ),

      // Setting fontFamily for the bodyText1 text style, that is used by default for the AppBar title and also for TabBar label if you use DefaultTabController.
      headline6: TextStyle(
        fontFamily: 'YourAwesomeFontFamily',
      ),

      //Setting fontFamily for the bodyText1 text style, that is used by default for the TabBar label.
      subtitle1: TextStyle(
        fontFamily: 'YourAwesomeFontFamily',
      ),
    ),
    appBarTheme: AppBarTheme(
      // Setting fontFamily for the AppBar title text style.
      textTheme: TextTheme(
        headline6: TextStyle(
          fontFamily: 'YourAwesomeFontFamily',
        ),
      ),
    ),
    tabBarTheme: TabBarTheme(
      // Setting fontFamily for the TabBar label text style.
      labelStyle: TextStyle(
        fontFamily: 'YourAwesomeFontFamily',
      ),
    ),
  ),
  ...
);

快乐编码...

我找到原因了。
Text小部件的TextStyle具有inherit属性。
如果inherit是false，它将覆盖TextStyle，但默认值是true。

text.dart

/// If non-null, the style to use for this text.
///
/// If the style's "inherit" property is true, the style will be merged with
/// the closest enclosing [DefaultTextStyle]. Otherwise, the style will
/// replace the closest enclosing [DefaultTextStyle].
final TextStyle? style;

但是AppBar的主题应用方式不同，它总是覆盖。

app_bar_theme.dart

/// Overrides the default value of [AppBar.titleTextStyle]
/// property in all descendant [AppBar] widgets.
///
/// If this property is specified, then [backwardsCompatibility]
/// should be false (the default).
///
/// See also:
///
///  * [toolbarTextStyle], which overrides the default of [AppBar.toolbarTextStyle]
///    in all descendant [AppBar] widgets.
final TextStyle? titleTextStyle;