The film was not submitted to the MPAA and was never rated. The "X" rating was self-applied by the film's distributor as a promotional device.
The film was made under the working title "Cheap" because it was a cheap little movie. The final title was "The Down and Dirty Duck", but Roger Corman (whose production company was distributing the film) released it with the short title "Dirty Duck" on all posters and advertisements to cash in on the recent trend of X-rated animated films which started with Fritz the Cat (1972). The title was never changed on the film itself was never changed.
Had the film been distributed under its original title, "Cheap", the film would have been advertised as "Roger Corman's Cheap!" When Corman realized that the title was a shot at his production techniques, he asked that the title be changed.
Despite the alternate title, this film has no connection whatsoever with the comic-strip character "Dirty Duck", created by Bobby London.