In terms of what really matters, he's a great parent. He teaches his children core values of honesty, kindness, and integrity.
However, some in the community might say he's a bad parent because he breaks society's conventions, including those such as racism, and does not always follow Southern traditions.
Well i think you have two good answers already. While im not sure that i agree with the statement that Atticus isn't a good father, you could certainly argue it with claims like you have above (leaving the upbringing to Calpurnia, not affectionate) you could also add that Atticus is distracted most of the time with his court cases. He pays more attention to his work, than he does with the children. His work also has negative affects on the children. One has to look no further than the public scrutiny put on his family. And of course, the attack by Bob Ewell was because of Atticus and his defense of Tom Robinson. Atticus almost lost his children because of his work.