In case of Silverlight application, you ll have to consider following points:
JMeter basically tests performance of server by testing backend services. It has nothing to do with front end operations performed by AJAX or similar technologies.
Silverlight is a front end technology and silverlight application mostly perform most of operation at front end.
So considering above points, if your Application is communicating with backend server; then you can definitely load test it (within scope of only those services).
Another challenge while testing silverlight applications is monitoring/recording of traffic. If JMeter fails to record application traffic to server then you can use Fiddler to assist you in this case.
In short, yes Silverlight application can be be tested via JMeter but there will be lot of limitations while doing the job.