You can use filters to move the add more button to the top of the group:
add_filter( 'rwmb_FIELD_ID_begin_html', function( $output, $field ) {
$output .= RWMB_Clone::add_clone_button( $field );
return $output;
}, 10, 2 );
add_filter( 'rwmb_FIELD_ID_end_html', function( $output, $field ) {
return RWMB_Field::call( 'element_description', $field ) . '</div>';
} );
Note that this doesn't change the behaviour of the button, e.g. when clicking on it, the new clone will be added to the bottom of the group.