If you are making a realistic cat puppet, then of course your head shape has already been defined. I'd use tons of reference photos of a single cat for a project like that.
However, if your cat is a more puppety (cartoonish) character, use a head shape that best represents his personality. Just some suggestions:
A sideways oval or football shaped head would be good if your character is a cool cat, or sly cat, or a girl cat.
A round head would probably suit a cute cat or kitten, or an "everyman" cat like Felix or Heathcliff.
A pointed head could be used for a cat character who's not too bright.
Of course, a lot more thought should be put into it. Those are just some preliminary suggestions. But the point is, your cat should look like his personality, whatever that my be. And that visual clue to his inner character can start with something as simple as his head shape.