This is a really cute pattern!
I think you could just add 2 stitches to the ears..one is right where your green square is and one right above it. And then repeat on the other side. That fills in the ears a bit. I think everything else looks fine.
You could try putting the whiskers coming from inside the face circle too but I think that your head is maybe a bit too short in length to do that. If you made it a bit bigger it might work.
I agree with a pp too though...I think if you embroidered the ears and whiskers it would look really nice and if you did a different colour they would really stand out.