The tough part of your question is perhaps the word "meaningful". There is no doubt that the Yugoslavian government was ineffective, greatly hindered by ethnic and other divisions, when it began to negotiate with the Axis for neutrality. When the Tripartite Pact was signed, there was a coup and the military took control and refused to say whether they would honor it or not, even though they were very badly equipped to resist an invasion. Hitler was furious, and invaded. That invasion took very little time- a little over ten days- and resulted in the quick surrender of the Yugoslavian Army. A guerilla war immediately began, which also was a very divided, chaotic affair, putting Serb "Partisan" militias against Croatian "Chetnik" militias, Partisan militias against German troops, Chetnik alliances with Germans. They were an annoyance to the Germans, but could not hold territory from the Germans- the German occupation continued into 1944, when pressure from the Russians in the East and the Allies in the West made it easier to dislodge the Axis. When the occupation ended, Tito and the Partisans were on hand to be able to quickly take control of the country, which was a good thing. But it's hard to see how they ever were a huge threat to the Wehrmacht.

Meaningful....well, there seems to be a great deal of romance surrounding the guerilla war, with iconic photos of beautiful, smiling women Partisans in uniform. And it would be possible to make a case for the existence of the Partisan Army at the end of the war ensuring that some sort of stability returned to the country, and perhaps the myth of the heroic fighter was a meaningful thing to those who had to rebuild the country after the war. But John Keegan, in his The Second World War, is scathing as to the nobility of it all. The guerillas took huge casualties: around 300,000 killed. Worse, the only defense a guerilla has is the ability to hide, usually among a civilian population. Unless the occupying army is exceptionally polite, it almost inevitably begins to attack civilians who, because they are not an army, can't defend themselves. There were close to 600,000 civilian deaths, including almost the entire Jewish population and a number of famous ethnic massacres, like at Kragjevic. While it's risky to engage in counterfactuals, it's hard to avoid concluding that the Yugoslav government should have actually gone along with the Tripartite Pact, at least at the beginning. And if it could not, it should have mounted a credible defense of the country. If it could not do that, if it had simply submitted to the occupation until the Russians and Allies had defeated the Axis it could have saved a million lives.