Is Silverlight Secure?

If you are asking what I think you are asking, no anyone can use something like dotPeek or Reflector to see your code. To make things harder, you would need to obfuscate your code.

Everything is insecure if you are not providing security to your apps.
http://stackoverflow.com/questions/2478230/how-can-i-protect-my-net-assemblies-from-decompilation
http://stackoverflow.com/questions/7314704/how-can-i-protect-my-c-sharp-code-from-being-decompiled
http://stackoverflow.com/questions/1276237/how-to-preventing-decompilation-of-any-c-sharp-application