Hi there,
ok if you select the Headline Block in Spacing settings you can set the Bottom Margin to another value. Otherwise it will attract the bottom-margin from the theme Customizer > Typography settings. If you set it to 0
then the space above and below the text will be roughly the same.
If you want to center the text, with the Headline block selected you can select Align Text > Center in the Toolbar.
If you want to apply that change ONLY to Mobile, then first set the Preview to Mobile, the Blocks Toolbar will move to the top of the editor, now when you set the Text alignment it will apply just to that screen size.