Nedic was a puppet. Serbia was an occupied state, just like Denmark or Poland. It was not a Tripartite pact axis member. Unlike Hungary, Romania, and Bulgaria, which were. Albania was Italy's puppet and Croatia was it's own breakaway fascist state.
Serbia was under direct German military occupation, and pieces were chopped off and given to greedy neighbors Croatia, Hungary, Bulgaria, German minority in Banat, and Italian-occupied Albania.
Nedic did cooperate with the occupation to a degree. He called his government the "government of national salvation" because his goal was to save as many lives as possible, not wishing to repeat the losses of WWI. In WWI, Serbia lost over 25% of the population (more than 1.5 million people) due to the Austro-Hungarian, German, and Bulgarian occupations, their atrocities, and the famines and diseases that spread throughout Serbia as a result of the war.
Is it possible that Nedic's police collaborated in arresting Jews and Gypsies/Roma? Yes, there was some of that. But most were arrested by German soldiers and the executions were carried out by German Wehrmacht and the SS.
I don't know of any spontaneous Serbian pogroms against Jews or Roma like what happened in Romania, Poland, Ukraine, or even worse in Lithuania and Latvia.
Also, all the camps in Serbia were set up by the German occupation, and the largest ethnic group of victims were Serbs in all of the camps, although unlike Jews in these camps they weren't killed because of who they were but because they were in the resistance or they were killed as "punishment"/reprisal for German losses (100 Serbs for 1 German). In Sajmiste, the largest camp in Serbia (actually it was on NDH territory at the time, but run by Germans) about 8000 Jews and 32000 Serbs were killed.
All these attempts to portray Serbia as a rabidly anti-Semitic anti-Roma country that jumped into WWII and fervently tried to exterminate its minorities is just total nonsense and most likely Croatian/Bosnian Muslim/Albanian propaganda. In reality, it was the Croats and Bosnian Muslims and Kosovo Albanians that were joyously killing tens of thousands of Jews and Roma and hundreds of thousands of Serbs in WWII.