Games no longer appear to be using Retina graphics
It seems the Angry Birds Seasons you're trying to play, doesn't have retina support. If you take a look at the customer reviews, you'll see some people complaining about the lack of retina support.

And if you're used to an iPhone 3G screen and make the switch to the gorgeous retina display, you'll see a huge difference. But so far, no retina support for Angry Birds Seasons. 

UPDATE:
It looks like Rovio is adding Retina support  to his latest creation (Space), so I'm guessing it won't be too long when the others games will support the Retina display too. 

(Or maybe the opposite, they don't add Retina support in order to convince the users to buy the latest Rovio products...)