Can passengers with reduced mobility travel on SJ night trains?

Yes. Yes, a wheelchair-accessible couchette compartment with sanitary facilities is available for passengers with reduced mobility: 

These couchette compartments offer space for one passenger with a wheelchair and an accompanying person.

Please reserve this compartment in good time at a DB Travel Centre, a travel agency with a DB licence or by calling  +49 30 2970 (charges depend on provider).