Remove ‘default’ font option from Gutenberg typography settings

You haven’t set the font in the block editor, instead you’ve added an additional font family option!

Instead use global styles instead to style the text:

  "styles": {
    "typography": {
      "fontFamily": "'Mulish', sans-serif"

Now the default will be Mulish, additionally setting fontFamilies to [] in the typography section will remove the font family picker