Mac OS X Mavericks Changes Default Font of Online Math Expressions Generated by MathJax
I believe you need to install the STIX fonts mentioned on this page.


  In Mac OS, open the Font Book application (it is in the Applications
  folder). Drag the fonts from their archive folder onto the “All Fonts”
  collection at the top left of the Font Book window. If you want to
  install them for everyone on your computer, drag them to the
  “Computer” collection instead. Alternatively, you can double-click on
  each font and select the “Install Font” button in the font window that
  appears.


*If that happens to be the ugly font you're referring to, then I would presume you could uninstall it, or disable it, and MathJax will use another (default) OS X font in place of it.


  See this question/answer for details.


There is also a fix listed here that will allow MathJax to use TeX fonts instead of STIX.