Backpressure is always a negative, so the muffler is a good thing. It will increase fuel economy.

Some people confuse backpressure with the effect various diameters/lengths of pipe in the system can have on tuning (essentially using the rest of the system to tune a header or exhaust manifold that doesn't give the desired characteristics on its own). That's due to flow rates and the mass of the gasses moving through the pipes, and it's real. However, if someone tells you, "Backpressure is better for torque," and you'll lose low end by running a non-restrictive muffler, that's plain wrong.