I personally think Tails' face looks fine. I don't understand why everyone is hating on it so much. It doesn't really look like a smile to me either. The moment I first saw it I recognized what expression it was supposed to be which has already been explained in earlier posts above. Sonic is heavy and Tails is making a strained expression as he struggles to carry Sonic. It's a more cartoony and exaggerated expression compared to the one seen in Sonic 3. I think it's fine. In fact, I think it's cute and also funny.