The fonts that come with the Mac distribution of GIMP are pretty horrible. If you use them, they all look really pixelated. You can quickly update GIMP to use the fonts native on your Mac. In GIMP 2.6.x this is done by going to Edit -> Preferences. Expand the Folders section in the left pane and then click on Fonts. Click on the existing fonts path and delete it. Then add a new font path to ~/Library/Fonts and click OK.

I found that I then had to quit GIMP for the change to take effect. But after making this change, GIMP then used the native fonts on my Mac and the text created in GIMP looked a lot better as a result.