From the Release Notes: https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-SW680
Text and font rendering on OS X v10.8 is optimized for Retina display. On a non–Retina display running OS X v.10.8, some font configurations can appear blurry in Xcode. 11486875
Switch back to non–Retina display optimized text and font appearance in Xcode by enterin this command in Terminal:
defaults write com.apple.dt.Xcode NSFontDefaultScreenFontSubstitutionEnabled -bool YES
It worked for me on Mac Pro running Xcode 4.4 on Mountain Lion.