This blurriness extends to album art as well. See the two attached snapshots of the same album (sizing isn't perfectly matched but very close). The first from MC, the second from iTunes. Both from a Macbook Pro with Retina display.
There are two issues you're seeing there:
1. Media Center uses a ton of JPEG compression for their thumbnails, whereas applications like iTunes do not - I think they're actually using PNG. I've been complaining about the quality of thumbnails for a while now -
just brought it up again recently.
2. Media Center does not currently have retina support. As I understand it, the way OS X handles things is that a 13" MacBook with a 2560x1600 Retina display reports itself to applications as a 1280x800 display at 2x scale.
Non-retina applications render 1280x800 @ 1x, and OS X scales that up, resulting in a blurry image. Applications with retina support render 1280x800 @ 2x (2560x1600) and OS X does not scale the image.
Media Center recently added in the option to scale up its UI to 200%, but it does not have Retina support
yet.
If I override my MacBook Pro's resolution from being 1280x800@2x to 2560x1600@1x with
DisplayMenu, and set Media Center to 200%, it looks
really nice.
Some of the graphics have not yet been updated to retina-quality assets, but it's still much better than it was before.
Unless there's some hurdle I'm unaware of, it seems like retina support is probably not that far away.
Here's a quick comparison: