It should work but, it could be a little simpler. $mode is already set for you by MODX, and since there are only two possibilities for $mode, the "else if" can just be an "else". Also, if the plugin is only connected to one event, you don't need to get the event name in the plugin, and if you're not doing anything when updating a user, you can skip the "else" altogether:
<?php
if ($mode == modSystemEvent::MODE_NEW) {
/* new user created *./
/* do something */
} else {
/* Updating existing user */
/* do something else */
}