A country's form of government does determine its economic success. When a country's government is more open and receptive to what its people have to say, like in a democracy, people will want to work harder, meaning its economy will be better off. If the country has an oppressive government where no one will have a say any where, it is hard to work, and thus the economy tends to be weaker.
Empires: Rome was very strong economically in the 1st century, but was weak in the third.
Democracies: the U.S. Is very rich, but Haiti is poor
Noble republics: Venice was super rich a tone point, but Was poorer later on.
Monarchies: Spain was super rich at the time of its colonial expansion, but Swaziland is a third world country.
Dictatorships: Burkina-faso is super poor, but nazi Germany's was a leading economic power at it's beginning (not saying they were good)
Communist nations: compare China to Cuba
Many countries with the same form of government have different economic outcomes. For instance, after the war, Iraq was set up as a democracy, yet the economy of the country is struggling. Other, less free societies such as China are doing quite well economically. Economic success depends on many factors, not just politics. These include infrastructure, monetary policy and social institutions, just to name a few.