Perhaps the question, "are sovereign citizens criminals," needs a finer definition before we can answer it.
The Federal Bureau of Investigation classifies "sovereign citizens" as criminals and subjects of an intensive counterterrorism campaign. Read more HERE. I'm not going to cherry pick the FBI definition, but the use of 'sovereign citizen' to define people who engage in fraud and other criminal behavior is unfortunate.
The US Constitution is the law of the land. That document provides various levels of sovereignty to the Federal Government, through the Tenth Amendment to the State Governments (ignored by the Federal Government) and to county, city, township and village leaders. Each has a jurisdiction or sovereignty accorded to them.
The English Common Law adaptation of 'every man's home is his castle' was particularly applied by the Fourth Amendment and the right to keep and bare arms was accorded to allow each citizen to protect themselves as necessary. Those are aspects of personal sovereignty. All Americans are sovereign citizens.
Thus we return to the 300,000 Americans that the FBI says are either hard core criminals or are courting the idea. Are Constitutionalists all criminals? I'll let you slice that any way that your conscience allows you to. People are best defined by their actions. Those who prey on the weak or disobey for the sake of criminal conduct are criminals. Those who have a moral affirmation to their nationality or their oath of office may not be.
One litmus test that seems to be commonly applied to the 'sovereign citizen movement' is that they don't want to pay taxes - and avoid them where possible. If that is the test, then there are a lot of progressives (including the Secretary of the Treasury), corporations and Democratic Party contributors who should be lumped in to the mix.
Perhaps the US Department of Justice and US Department of Homeland Security could find a better common name for extremist criminals other than loosely tagging them all as "Sovereign Citizens".