Is it well-received to thank the assignment author at the end of a course?

Yes, I think it would be fine. Everybody likes to know their work is appreciated. Teaching assistants are often not appreciated enough by their students (and sometimes not enough by their professors!).

Just a short concise form of thanks would be enough:

Thanks for those exercises you set for course xyz. They were really interesting, and I enjoyed solving them. I hope you liked creating them as much as I appreciated their contents.

Something along those lines perhaps. If you make it too long it might sound obsequious or sycophantic.

In Germany

From my point of view, being a German computer scientist, I would also deem it appropriate to thank the team. From my point of view it is perfectly fine.

In General

Most certainly it would leave me smiling for the rest of the day. Everybody likes to hear that he/ she did well, as Brian already wrote.

Feedback

To give another angle why this is ok: Let's think about Feedback.

When designing courses and assignments, feedback from the students can be extremely helpful. It is not easy to judge how an assignment is received and nobody learns from assignments that are way too hard or way too easy. So if you tell them the assignments worked well you are providing important information to further improve the class.

In my experience, negative feedback is more readily given than positive. On the other hand, it is often not useful hearing that the course was too hard, from someone who skipped half of the sessions.

That said, I think your point of view matters here and can help shaping the next iteration of the course. Share it.

My opinion is that if you feel thankful toward someone for something, just express it. Sometimes it might seem awkward, but so what? The thanks can be very simple, or more elaborate, depending on the size of the act, among other things. Don't overthink it.