Can I increase the default font size for the Notes in Mountain Lion?
You can go in "Terminal.app" and write (if your system is in english):

nano /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist 

You will see a long file, but it's easy to understand it's content. You just need to change the ?? value, it is the default font size for each font.

When you are done, just control+O to save then control+X to close.

Restart Notes, select your default font and your font should follow the new default