Its effects are hard to disentangle: expansionary fiscal policy boosts demand in the short run, but might reduce expected future growth in the long run if people focus on the need to fund current fiscal spending. That said, if fiscal policy is persistently tight, it is likely that monetary policy will have to be persistently accommodative to offset the contractionary influence on the economy.
And if your model of the neutral rate does not explicitly include the global stance of fiscal policy, it will interpret this as the neutral rate having fallen. This omission might go some way to explaining a puzzle surrounding standard estimates of the neutral rate.