Insert After Each Label in Radio Box Group

I think the best way to do that is via CSS. You can install a plugin like Add admin CSS. The CSS to make each label display in ins own line is quite simple:

.rwmb-radio-wrapper .rwmb-input > label {
  display: block;
}

If you don't want to use a plugin, you can enqueue a CSS file on Add new/Edit page with the same snippet above.

This way you also can add more margin to label for a better look.

But if you have to use PHP to output line break after each label tag, please use rwmb_radio_html filter, like this:

add_filter( 'rwmb_radio_html', function html( $output, $field, $meta )
{
	$html = array();
	$tpl  = '<label><input type="radio" class="rwmb-radio" name="%s" value="%s"%s> %s</label>';

	foreach ( $field['options'] as $value => $label )
	{
		$html[] = sprintf(
			$tpl,
			$field['field_name'],
			$value,
			checked( $value, $meta, false ),
			$label
		);
	}

	return implode( '<br>', $html );
}, 10, 3 );