Nios is based on MIPS architecture and it is totally different from DSPs. If you need a high performance Digital Signal Processor, you may use an external DSP or implement your processing inside FPGA using hardware logic (not software-based Nios application). Architecture of Nios is for embedded applications, not DSP applications.
To reiterate in different words; NIOS is a microcontroller, like the PIC series from Microchip or any of the ARM devices. A DSP is, as defined by its name, a digital signal processor and not a microcontroller.
You are perhaps interested to know about not exactly DSPs but rather DSCs, which are unities which lies in the middle of the two worlds. They aren't so efficient like powerful DSPs, but has the standards peripherals integrated on chip together with an ALU a bit more capable than those we can find on standard microcontrollers.