rhetorical: rhetorical speech or writing is intended to seem important or influence people
A slightly wider definition is:
rhetorical: A rhetorical question is not a question about the art of speaking effectively; it is a question that is asked for effect, rather than from a desire to know the answer. “Would it kill you to stop chewing your food with your mouth open?” is a rhetorical question.
Flab = noun (informal, disapproving): soft, loose flesh on someone's body>
By analogy, flab is useless words, phrases or sentences in prose. It is not only useless but, like body flab, by the weight of its presence it imposes a burden on the understanding of the substance of the text.
We may understand rhetorical flab as unnecessary and burdensome content that is pretentious and tries to impress by mere form, convention or cliché rather than by content or meaning. It confuses and distracts from meaning.
When the basic message is simple or weak, we may describe it as brittle-boned, and therefore weakened rather than strengthened by the surrounding flab.
Here is an example from our own British prime minister: at one point in a speech he is uttering a simple and rather banal Covid message of "We are worried about jobs, businesses and the economy and we should plan to mitigate the effects", but he pads it with flab and says instead:
...and we know that people are worried now about their jobs and their businesses and we are waiting as if between the flash of lightning and the thunderclap with our hearts in our mouths for the full economic reverberations to appear and we must use this moment – now – this interval to plan our response and to fix of course the problems that were most brutally illuminated in that covid lightning flash"