Hiding Jetpack Ads from users who are signed in?

Your code should be the good one. You just have to use is_user_logged_in() into a hook instead of calling it directly, and probably to early.

function wp_se_339916() {
    if ( is_user_logged_in() {
        add_filter( 'wordads_excerpt_disable', '__return_true' );
        add_filter( 'wordads_content_disable', '__return_true' );
        add_filter( 'wordads_inpost_disable', '__return_true' );
        add_filter( 'wordads_header_disable', '__return_true' );
    }
}
add_action( 'template_redirect', 'wp_se_339916' );

Using the template_redirect hook could be a good solution. You’ll just have to paste this into your functions.php file. https://developer.wordpress.org/reference/functions/is_user_logged_in/