The Tsar Bomba was clean because they set off only 2 of the 3 stages. It had the fission primary and fusion secondary, but had a steel/lead casing rather than the U-238 casing it was designed with, bringing it down from possibly 150MT to the 50MT we saw.
They didn't want to poison half their arctic forever just to impress us.
And the plane that dropped it barely got away as it was. Dropping the 150 version was probably going to be suicide.
The full bomb would have been very, very dirty. And the cobalt variant would have been dirtier still.