'Postmodernism' is a complex phenomenon. So complex a phenomenon it is entirely doubtful whether it is a unified phenomenon at all... which, ironically enough, is an entirely postmodernism-compatible observation.
In a narrow sense, I am against it. Of course I am. I'm a Marxist. As such I am, naturally, 'against' any theoretical approach to the world that isn't Marxism, in the same way that any non-Marxist is going to inevitably be 'against' Marxist.
Much (most) of 'postmodernism' is anti-Marxist. Much of it is reactionary in many key respects. Much of it is progressive in some senses. It has its excesses and follies, its uses and insights.
What it *isn't* is a dogmatically accepted prevailing orthodoxy that undermines our sense of what is real via an assault on empiricism in education, media, etc.
Both education and media are quite capable, as branches of bourgeois society, of juggling empirical accuracy and ideological deception as required by the capitalist system, without needing French theorists to tell them how to do it.
Anyone who tells you there's a discrete and unified 'thing' called postmodernism that rejects all objective truth, rules in academia, and thus generates poisonous wokeness and fake news, is quite simply selling you snake oil.
And I say that as someone with very little time for, say, Foucault, who I consider the second most influential anti-Marxist thinker of the 20th century.
(The fact that postmodernism is built on a rejection of grand narratives like, crucially, historical materialism, is an immediate and crude - but entirely adequate - refutation of the nonsense you get from people like JB Peterson, with their "postmodern neo-Marxists.)
The most influential was Hayek, and his influence was vastly greater precisely because, rather than undermine historical materialism in academia, he intervened directly in politics and economics.
If you want to look at the real phenomenon of a powerfully anti-empirical and relativist ideological trend in academia that really *has* changed the material and social world we live in, I direct you to Hayek and other reactionary economists, and their legacy:
a vast network of billionaire-funded non-profits and think tanks and policy institutes and lobby groups, nestled within academia, and wielding enormous influence throughout the late 20th century and into our time, all founded on the intellectual legacy of right-wing academics...
...who were creating the legitimising ideologies of neoliberalism, not discussing the post-colonial ramifications of Wuthering Heights or doubting external reality.