Kirby's angry look was implemented to appeal more to western audiences. Apparently we are more likely to buy a game with an angry, aggressive looking hero than just a cute and cuddly one. It doesn't really do much to stop his cuteness though.
You'd be mad too if some unknown force keeps trying to take over your universe. You'd be even madder if someone was stealing your cake. Kirby has every right to be mad and I don't blame him.
