How to turn on/off fonts anti-aliasing in Notepad++?

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.

Settings > Preferences > Editing > Enable smooth font.