How to dequeue the default CSS styles on the wp-login.php page?

Yes, it’s possible, You can use the wp_dequeue_style(), you just have to pass the handler ‘login’.

function wp_stackexchange_267327() {
    wp_dequeue_style( 'login' );
}
add_action( 'login_enqueue_scripts', 'wp_stackexchange_267327' );