The User:isaacl/script/diff-mono-font.js script configures a specific monospaced font to be used when displaying diffs. (The specific font used is subject to change based on my personal preference.) You must download and install one of the fonts listed in the corresponding stylesheet file; otherwise, your browser will use the font it has configured for monospace text.
Instead of using a user script, you can easily modify your own common.css stylesheet to include the contents of User:Isaacl/style/diff-mono-font.css, or to import your own stylesheet. The only advantage to using a script to load the stylesheet is to allow it to be managed by the script installer gadget.
As noted in Wikipedia:User scripts § A word about security, if you choose to import this script into your own personal Javascript files, then you are giving the script the ability to execute code in the context of your account. This requires that you trust me as the author for any future changes that I make. I strongly suggest that you instead decide if you trust the current code, and if so, copy it into your own Javascript file, as well as the stylesheet into your own stylesheet.