Perhaps the issue is that, like so many science fiction authors, his ideas are interesting but his actual writing, characters and dialogue have nothing in particular to recommend them. In short story form, the ideas come and go quickly enough that one doesn't have a chance to get too bored with them. In a novel like Solaris, on the other hand, there's really only one idea and it takes a hell of a lot of words for it to actually get out there.

Asimov is similarly bad at actual writing, but his ideas are sufficiently good and they come sufficiently thick and fast even in his novels that he's still pretty okay to read.

In contrast an author like Neal Stephenson manages to write a science fiction story with proper characters who are interesting to read about even when they're not expounding on some science-fictional idea.

It's harder to tell how good an author's writing is when it's in translation. The Cyberiad's writing was absolutely fantastic IMHO. But I can't say if the Polish original was that good. Lem's longer, more serious novels don't sparkle like that, at least in the English translation.

True, though good writing isn't just about good prose and putting well-chosen words in an appropriate order, it's also about more macro-level structure and tension and pacing and characterization.

Good point. However, would you say that Dostoyevsky has tension and pacing? I raise him because Lem does, and to my mind, Lem reads more like Dostoyevsky than like Stephen King. How about Dickens or Victor Hugo?

I admit that I don't know enough (I fail at reading Dostoyevsky) but it is counted as great writing despite lacking in the pace that modern readers desire.