Well, Marxists certainly don’t believe that rhetoric is king - rather the complete reverse!
When you say “PoMo” what is it that you’re talking about? The trouble with these claims is that many theorists of the postmodern argue that postmodernism is not a position, but a state of late capitalism or a stage of history. They are largely critical of this - and precisely through a cultural-materialist or Marxist lens (eg coming out of Frankfurt School Marxism, which is highly critical of the nihilism inherent in society’s move towards postmodernism).
Marxists view postmodernism as a catastrophic stage of late capitalism, where we internalise the nihilism of capitalism to a degree that it is impossible properly to understand our own alienation.
The classic poststructuralist theorists of the postmodern were also deeply critical of it as a negative cultural force - they were diagnostic of it rather than approving of it (eg in Lyotard’s foundational The Postmodern Condition). They also saw postmodernism not as a belief you can adopt; but a malaise, a stage in cultural history with malign consequences.
So I’m interested in the reduction of these positions - often extremely nuanced and historical - to “PoMo” — because one of the other things about the postmodern is that it has a huge range of meanings (postmodernism in architecture means something completely different to in literature or in intellectual history, for example). It’s a term with a big range of histories and sometimes contradictory definitions, so it would be helpful to specify what it means in this context, because it’s widely thrown around without much clarification - often caricatured and lots of times inaccurately so.