Dream sequences aren't always valid crossovers for the Toobworld Dynamic. With some, as when TV characters dream of interactions with the characters from 'Gilligan's Island' ('Meego', 'Roseanne', 'Alf', 'Baywatch', and even in the Land of Remakes with 'The New Gidget'), there is no Zonk because that TV show was based on the true events dealing with those seven stranded castaways.
But in other cases, we have to consider those dream sequences to be Zonks:
Don't get me wrong, I believe Leon the Lizard and Digit are living puppets in the world of Earth Prime-Time, but I'm not sure if Dr. Huxtable would be aware of their existence. One way in which that could work is due to a segment of 'The Jim Henson Hour' called "MuppeTelevision", which served as a 'Muppet Show' continuation, after a fashion.
Perhaps Cliff Huxtable saw that independently of the rest while watching TV with his youngest daughter Rudy.
That just might work. Otherwise, it would have to be considered a Zonk.