Assuming that Stuxnet was designed to delay Iranian nuclear efforts, there are different degrees of "working". Delaying for a week is better than delaying for a day, etc. If better cloaking would have allowed Stuxnet to remain hidden for longer, than it would have delayed Iranian nuclear efforts more.
Sending a clear message of, "we have the ability to launch effective cyberattacks against you" may have been a goal as well. Make it too stealthy and it might be a while before anyone realizes they were attacked.
