It seems clear to me that the animal and fantasy characters in the Shrek films are much more emotive than the human ones. It seems odd that this was not clear to the film-makers, especially as the crux of Shrek 2 turn Shrek himself into a human for a small period. The design for the humans seem overly stiff, unable to express much more than the lines their celeb voices are reading. Wheras Donkey and Puss In Boots show the full range via body language as well as a more gung ho physicality. The upshot of this is that the voices match the bodies, whilst you are acutely aware of the celeb voices hanging above the human characters, so much so that the John Cleese seems rammed poorly into his shortish king character and is only unleash at the denoument.
The lack of care between voices and casting seems to hover over Princess Fiona in particular. We only just get used to her mooning and moping in ogre form, and then she gets put back in her old body where she pretty much continues to frown. Mike Myers on the other hand manages to just about pull of the neutered Shrek by making the oft mistaken (but not here) link between acting and doing a voice.
Shrek 2 is plenty of fun in places, but holds together less than the original as a story (ie its the same story in reverse with a different baddie). It is a pity that the scriptwriters, in finding genuinely clever fairy tale ways to vanquish the bad guys did not apply the same intelligence to the overall product. With the status quo at the end of the offensive original, Shrek 2 could have happily been filled with no human characters at all.