The Fiat 124 was actually a pretty good car for its era. Russians improved its suspension, refitted the engine, and messed up the hydraulics. Still, pretty good car for the 60s. And then, they continued to produce the same car with miniscule modifications until 2010s.
That's the problem with authoritarian regimes. You can buy a plant by a fiat (pun intended), but you can't make a decent car by a decree.
They could have done much better if it was a priority. The priority however was tanks, nuclear submarines and missiles. Up to 25% of GDP was outright wasted on the military.
Not within the socialist system, there was absolute zero incentive to do a quality job. Sometimes there were incentives to do more on the quantity (see "udarnik") with moderate success but these were detrimental to the quality.
That's the problem with authoritarian regimes. You can buy a plant by a fiat (pun intended), but you can't make a decent car by a decree.