Is there a way to turn on/off anti-aliasing in Notepad++, either through settings or a certain plug-in?
Notepad++ uses the same settings as the rest of the applications on the system. There isn't any setting within it to change this.
P.S.: If you are having issues with blurry text on DPI settings higher than 96 DPI on Windows Vista/7, turn on "Disable display scaling on high DPI settings" on program's "Compatibility" tab.