Toy Story 3
I haven't seen any of their films since Up, so I can't really comment on their recent output's quality, but reviewers have certainly noticed a decline.
got as much praise as any Pixar movie ever has. Cars 2
got scathing reviews, absolutely, but that's John Lasseter's pet enthusiasm, not something Disney forced on them. Brave
got better reviews than Cars 2
(it's not their best work by any means, but it's not bad either).
I don't see any real basis for the idea that Disney's acquisition has hurt Pixar. John Lasseter is in charge of running both now.