I wouldn't use the second one without more, as in: He is not good at showing his emotions. He is not good at parallel parking. He is not good at managing his time.
The only time I could imagine using it is something like "She plays the piano so well. She is so good. He... well, he is not good. But he tries hard." Even then, the meaning is "He is not good at playing the piano."
Student or Learner