I don’t know if the raspberry pi can be used to develop jME games or not, but as long as it’s running Linux I don’t see why it wouldn’t. Note that this has a serious power drawback, however - a raspberry pi does not have nearly as much compute power as a half-decent desktop.
As far as using it in a cluster, I’m not quite sure what you mean. It is straightforward to write a game server in stock jME, and it’s also straightforward to write your own server heartbeat loop and use only SpiderMonkey without the rest of the engine. As far as true “clustering” behavior (work distribution/sharding/failover), that’s either up to you to write yourself or use some other framework for.