I have an outstanding pull request where I fixed the monospacing issue on OS X, although I added it in a bigger pull and forgot to mention it now I looked it up.
Anywho, if you don't want to wait you can hack this in.
lib/gui_qt.py : Replace line 48 with
49 MONOSPACE_FONT = 'Lucida Console'
50 elif platform.system() == 'Darwin':
51 MONOSPACE_FONT = 'Monaco'
52 else:
53 MONOSPACE_FONT = 'monospace'
I'm not 100% sure Monca is a normal system font so please let me know if that works for you.