I've been down the rabbit hole and I've found treasure!
It was deliberate. Benny the Ball was voiced by Maurice Gosfield (1913-1964), who played Private Doberman. Phil Silvers found him rather pompous and said he "thought of himself as Cary Grant playing a short, plump man".
Brain and Spook were both voiced by Leo De Lyon (died 2021 aged 96) whose party piece was singing one tune while whistling another. There is a clip of him playing piano with Phil Silvers but not doing funny voices. Here he is with Dean Martin, giving it everything.
As well as De Lyon, Arnold Stang (Top Cat) and John Stephenson (Fancy-Fancy) also lived into their nineties. Marvin Kaplan (Choo-Choo) was 89.
Dean Martin's reaction was pure gold! It's as if he had no idea what would happen.

