Is it appropriate to show concern for a professor's well-being? I am an student and in one of my college courses, the professor seems stressed or frazzled. He is always running late and unprepared, which seems odd because the Powerpoint slides are already done and he uses previous tests and quizzes. The tests, homework, and most of the quizzes are computerized and graded automatically. So, He can't be overwhelmed with grading. Which leads me to believe that he is having problems at home.

My question: would it be appropriate to ask him if he is okay, either by email or face-to-face? 

I don't want to invade his privacy but I don't like seeing anyone in distress.
He either has personal issues, or he doesn't focus on teaching.

If he is having personal problems, it is wrong to approach him like a friend and expect him to give details. You are the student and he is the professor, and his personal issues are private and he will share them with someone close if he wants to. As an adult, he probably knows how the process works.

Either way, the only thing that matters is whether he is doing his job properly. If you think he is not doing so, then you should give constructive criticizm, such as when you are in class and you don't understand, you can say that for the last classes you are having difficulties understanding and putting it all together and suggest a method so that you can benefit better from his course. Or, you can send an e-mail about how certain things can be improved without sounding very negative.