1stly curves are always gonna be a bit raggedy. just go over it like 10+ times. The slower you go/more times you go over it the better.
2ndly don't worry about those pieces.
that is what the curved surface of the ears looks like when layed out flat. You don't need to cut these out of card.
also I wouldn't use a powder and water mix wall filler. Use the stuff that comes ready in tubes, there's no danger of the cardboard becoming soggy that way.
and in answer to creon's question. When painting I wouldn't use a clear coat (not at first) . I just went straight in and used a white primer paint that said suitable for MDF. Anything that works on MDF or wood will work on card.
and there's no point trying to put a thin layer of filler on the whole thing. If you've worked carefully the best finish is achieved by painting primer straight onto the card.
hope this helps