The general idea is that a particular piece of writing can create vivid imagery and emotional impact for readers, or it can lose its power through repetition or simplicity.
Think of the expression "thinking outside the box". This was probably pretty powerful at one point! Then everyone started using it, and it became a useful tool that was no longer evocative. Everyone has heard it before, and it's a well-worn bit of text. Most of this is because its imagery has been replaced by meaning. You read that phrase and likely no longer picture the box, right?
Here's a quote I had lying around from Harold Bloom's review of Harry Potter:
Her prose style, heavy on cliche, makes no demands upon her readers. In an arbitrarily chosen single page—page 4—of the first Harry Potter book, I count seven cliches, all of the "stretch his legs" variety.
And this is generally true, Rowling goes heavy on stock phrases, many of which don't quite rise to the level of cliche. I'll part with Bloom here and say that I don't think Rowling has wholly bad prose in general, but she does like to lean on the stock, even if it's occasionally punctuated by a nice turn of phrase.
Compare this to a book that I somewhat liked, whose prose I thought was (overall) flatly good, The Ministry of Time:
Despite being out of uniform, he looked oddly formal, as if he was the only person in serif font.
This is a line that gets me to sit up, and the novel is full of them.
Distant conversation recalled the clatter of pebbles underfoot.
Isn't this nice? Doesn't this make you think, turn the gears in your head? It has an unfolding quality to it, which is something that, in my mind, "good prose" and good metaphors do well. There's specificity and force to it. The writing evokes things and makes connections in my head, and I read a little slower because of that, to give the disparate feelings time to sink in. To give a non-metaphor/simile version (still Ministry of Time):
As soon as she stepped in, I started making a tally of the evidence of unbeauty.
Not a particularly complicated sentence, but it's got unexpected qualities to it. You do need to think a little more about it, and can't just let it skitter across your brain. Compare to this randomly chosen piece from A Court of Thorns and Roses:
The sun had set by the time I exited the forest, my knees shaking. My hands, stiff from clenching the legs of the deer, had gone utterly numb miles ago.
I personally feel like relatively little of this is about clause complexity, which is (generally) more to do with the specifics of how a sentence feels to us and how well it fits into the patterns of phrases and sentences we know. It's hard to evaluate, because you're evaluating it against the canon of English literature, and I'm not sure there are concrete rules to follow here.
Technically you could make frequent trips to Google's n-gram viewer and get some concrete information about how stock a phrase is:
But this seems like it's going too quantitative and prone to errors.
A simple sentence can be arresting and a complex one can be utterly forgettable. My opinion is that this comes primarily from predictability, of which cliche is only one (relatively severe) aspect. It's whether our pattern-matching brains delight in a new connection or gloss the familiar.
There are readers who would prefer to not engage their brains all that much when reading, they'll readily self-report that. But there are ways that "bad prose" in my framework can go beyond that, becoming repetitive and low in information. There are people who don't just like comfort and familiarity, they like for their brain to be doing less work per word.
And this is one of the things that adjective-heavy "purple" prose is doing, in my opinion. If you take a sentence and add an "expected" adjective to almost every noun (e.g. "green leaf") then the sentence can be read faster, or the meaning of the sentence comes slower, and you can float along with the writing in an opposite mode to how I generally like to read, and how I generally model people as reading.