Mr. Knightley is not meant to be a hottie. He's 37 years old, compared to Emma's 21 years. The whole point is that he has to compete with the younger Frank Churchill, who is handsome, charming and well, younger. Mr. Knightley is supposed to the last man that Emma would think of to fall in love with in the movie, and he stands on the sidelines alot.
I don't think he is ugly, he is exactly as you would think a 37 year old should look like, especially in that era. His eyes are intense, especially when he looks at Emma, (i am referring to the 1996 Jane Austen's Emma version starring Mark Strong as Mr. Knightley).
Well, thats just my opinion.