Depends on where you live and the type of soil you have. For example Florida has sandy soil and they don't mulch there because citrus roots are very shallow. In other areas you can mulch, but keep the mulch 10 inches away from the tree trunk or it can cause rot to the tree. Pine mulch is best for citrus. You might also want to see if the tree needs some nutrients if you haven't done anything with it for a while.