Yes, you can connect the input ports of the System Maintenance Slave Interface to ground and leave the output ports unconnected, if you do not use the interface in your design.
However, if you disable the interface, you will lose the flexibility of programming the SRIO core's Command and Status Registers (CSRs). You will also lose the capability of monitoring the status of some of the CSRs. If your application is able to function with the default values in the CSRs, then you may safely disable the interface.
We currently do not have a figure for the number of LEs that will be saved by disabling the interface. You will have to do your own analysis as to the number of LEs that you will save by disabling the interface.