Some people are just naturally frazzled. They could be having the greatest and most relaxing day ever, and show up to class 4 minutes late, hair all wild and uncombed, wearing odd socks and having forgotten to bring you your marked assignments. They may be remarkably easy to frazzle or they may not even feel frazzled, they may just look it! Asking if everything is ok, especially if you say why you are worried and point out you see a pattern, will probably offend this person.
Or perhaps something really is bothering your prof. I use "the same slides" every year (except that I read them over, tweak them, add some, rearrange others etc) and reuse some aspects of the tests and quizzes. You may not know this, but that's not the hardest part of teaching. Nor is marking. And the only profs who teach a single course are adjuncts like me who have a whole 'nother life off campus, or super distinguished researchers who've had their load lowered. So you are not the only dance this prof is dancing, and you may not be the most important one, either. Your belief that the workload for this class is easy doesn't mean that the prof's overall workload is easy, by any means.
But hey, perhaps you're a very perceptive person (I'm not, but I know some) and you've nailed it: this prof is going through hell with something personal. I have had to show up and teach while going through hell (a dying parent, for example) and about the worst thing that could have happened is some kid (everyone under 30 is a kid to me) asking me if I'm ok and if there's anything they can do to help. Some kid who doesn't even know me! I'd be so humiliated that my distress had shown through. I get my support from a variety of people, and I choose who I want support from. I would have trouble even stammering through a sentence like "I'm sorry if my performance isn't up to your standard this week, I have a lot on my plate that I would rather not discuss."
I just can't see this question leading to a good place if it was asked of me, even if (and it's a big if) your assumption of a personal problem is in fact accurate. And if it's inaccurate, that's even worse. There's really no upside.
Here's the furthest I think it's ok to go. You're having the usual conversation that you do after class, with questions about your field etc, and the prof either flat out says "I don't have time to discuss this now, I have to go and deal with something" or you get that perception. You could carefully say something like "Sorry, I didn't realize you had less time than usual this week. Hope things let up for you soon." Most likely the prof will just grimace, say something noncommittal, and head out. But there is a chance you'll get a reply like "I hope so, I can't take much more" or "no, I'm afraid it's going to be like this for months and then it will get worse" and those are openings for you to say something pleasantly supportive like "oh dear, I'm sorry to hear that. How can I help?" But without that opening, even if it's obvious to you what's happening, maintain the fiction that it isn't. That can actually be a form of help.