There seems to be two theories on this as James (Mr Vacuum Cleaner) points out.
1. Customers suggested it to the company
2. "The name and the face were both Duncan's ideas, put there (in his charming account) because the lonely cleaning armies of the early morning and late night liked to use an object they could address as a friend." As we suggested here.
It was probably just a good idea someone at Numark had and someone internally just made some face stickers. We don't know who actually designed the eyes and the smile, and maybe we never will.