This is the default view of code in SharePoint Designer, using Courier New, 9pt:
Ugly. Hard to read. In my humble opinion.
To change the code view font settings, go to the File tab, then choose Options. Click the "Page Editor Options" button. Then select a different monospaced font. In the examples below, I chose Consolas and set it to 8pt. The font size change is a different issue, but I like it because I can see more lines of code on the same screen.
Now my code view looks like this:
Cleaner. Easier to read. In my humble opinion.
There are other fonts you can try as well. Just make sure you can easily distinguish the characters, such as "o", "O", "0" and "i", "l", "1". The following image shows five fonts set to 8pt:
Notice that Consolas is slightly skinnier, allowing you to see more code in less horizontal space. Also notice how clearly distinguishable the numeral "0" is from the letter "O". The lowercase "g" is a little weird compared to the others, but I can live with that given the other benefits.