The comparison between The Three Body Problem series and some of Le Guin's books is interesting because the two authors have such wildly different strengths and weaknesses. I love them both but, purely in terms of writing, there is absolutely no comparison. Le Guin is one of the all time greats .
Just to be clear, I liked the 3BP series a lot. The books are generally paced well and there's just about the right amount of world building. However, the long expository monologues break up the flow. It's a shame because it's a great story and he tells it well. Those who have read the books will recognize his over reliance on on a particularly lazy storytelling device to introduce ideas which I can best summarize as follows: "here is this idea and this is why it's important and here's all the nuances to it so then this next idea came about and this is why it's important." That's bad writing. Making the characters academics so that Liu Cixuin can dress up his ideas as "theories" in this way is wholly unconvincing and, as you said, amateurish.
By contrast, Le Guin is masterful at weaving her ideas into the narrative of the story. Ideas are not introduced as standalone concepts and then referenced by the story. Instead, they are woven into the fabric of the story itself. This makes the ideas more compelling and keeps the narrative brisk and interesting. On the other hand, Le Guin often spends far too much time world building. The Left Hand of Darkness is one of my favorite books, but the first half of the book plods on rather painfully. The second half feels like a reward for slogging through the first half.
Not sure if you've read The Dispossessed but I think it's better than TLHOD. It still has a bit too much world building in the first half but it's a tighter and more compactly told tale. Her most tightly told tale is probably The Lathe of Heaven. That book was incredible and I think it showcases her literary skill better than some of her more famous books. Definitely check it out some time if you liked her other work.