If we're looking at walls a strict "damage-taking" standpoint then yes, Cressy is one of the best walls. But in an actual battle, just being able to take hits doesnt help much. For example, as you mentioned, in terms of defense tiers/resistances what not, Cresselia should have an edge over Hippowdon. But the lack of ability to do anything other than attack (I'm talking about Restalking Cressy not Reflect/Twave versions) means that she's set up fodder. Hippowdon on the other hand can deal good amounts of damage with the attacks it has, unlike Cressy, and can also serve as a SRer, pHazer and sleep inducer (Yawn; although Yawn will probably be pHazing pokemon anyway).
Of course the Reflect/Twave (or Light Screen if you want) versions are much better overall in terms of usefulness. But with the abundance of SS teams running around, the lack of a reliable recovery move means Cressy isnt living for long. That's why Cressy isnt exactly one of the better pokes to use right now.