How bad is it? Barely adequate animation.
Should you see it? No. It adds nothing to the original.
I suppose the idea for this film was to introduce children to the music of Rogers and Hammerstein without their having to watch an adult romance with adult themes; it accomplishes that and nothing else. The animation is flat-looking and the changes to the script add a fire-breathing dragon that runs away from whistles, a comic foil who loses his teeth one at a time (a subject that I personally find terrifying) and an ending where the King doesn't die. The slapstick comedy detracts rather than adds. This features the voices of Miranda and Ian Richardson, Darrell Hammond, Adam Wylie and Kenny Baker, among others. It's not terrible by any means, just unnecessary.