Add OR in mu-plugin to check if one of multiple users is the logged in user

Your question boils down to simple PHP. It is basically how to avoid

'AdminUser' !== $user->user_login || 'AdminUser2' !== $user->user_login || 'AdminUser3' !== $user->user_login || 'AdminUser4' !== $user->user_login || etc.

One way to solve this is use in_array() instead:

$allowed_users = [
    'AdminUser',
    'AdminUser2',
    'AdminUser3',
    // etc
];
$user = wp_get_current_user();

if($user && isset($user->user_login) && !in_array($user->user_login, $allowed_users)) {
    echo '<style>.update-nag, .updated, .error, .is-dismissible, .notice.woo-permalink-manager-banner, #try-gutenberg-panel, span.update-plugins, .yoast-issue-counter, table.wp-list-table.plugins .notice-warning { display: none !important; }</style>';
}