Why does Dia interface look blurry on my Mac?
The issue stems from the way X11 handles pixels. There are some third party workarounds, but it would seem that one needs to wait until XQuartz is updated to deal with Retina scaling.

I have installed QuickResFree from the Mac App Store, which lets you have finer control over the resolution than the Mac display defaults. This makes it so I can use my X11 apps and decrease the apparent blurriness, but switch back to something more reasonable for Retina-compatible apps.

I will note that this solution is not optimal, and the problem still lies in the fact that many apps are not yet programmed to be able to deal with the pixel scaling and Retina.