Display Custom Text if date_picker date is expired

If you’ve dropped it right into functions.php and not inside an action hook, the code is running before ACF has initialized. Try running on acf/init action (untested):

add_action( 'acf/init', static function () {
    $currentdate = new DateTime();

    $date = get_field( 'date_picker', false, false );
    $date = new DateTime( $date );

    if ( $currentdate > $date ) {
      echo '(Deal Expired)';  
    } else {
      echo get_field( 'date_picker' );
    }
} );