Just wanted to +1 this as I also make code notes and get frustrated with Evernote over it, and want to add the following:
a) Using a convention like ` (backtick) to surround code blocks (in typed notes of course) would save a lot of faff with, as OP mentions, selecting text then changing the font
b) What's really, hideously annoying is that the Windows and Mac clients don't have a monospaced font in common! Ugh!! This means that
text formatted in Courier or Consolas on Windows shows up in Helvetica on Mac
vice versa: text in Andale Mono on Mac shows up as Arial on Windows
Monospace is monospace for a reason; it doesn't really matter which typeface, just please find a way not to convert text from mono- to non-monospaced font, it's so annoying.