Paul Krugman says: "Monetary policy at the ZLB requires a perceived regime change to be effective, fiscal policy doesn’t — in fact, fiscal policy works better if people believe that it’s temporary, and will end when the economy recovers."
In a New Keynesian model temporarily stuck at the Zero Lower Bound with less than potential output, a "temporary" increase in Government spending will increase output, but a "permanent" increase in G will have no effect. But "temporary" has a very precise meaning here. It does not mean what a normal person might think it means. If people expect any delay, however short, in cutting G after the economy escapes the ZLB and returns to potential output, the increase in G might as well be permanent. It will have no effect on output at the ZLB.