Anti-liberalism and Marxism may be popular and therefore people join in dismissing Marx, but afaik Marx was and to some extent is (the ideas are from a long time ago) taken seriously as a serious theorist.
I'm not sure whether to parse this as (anti-liberalism) and (Marxism) or as (anti-liberalism) and (anti-Marxism). But in case you meant the latter, Marxism is explicitly anti-liberal in the sense that he rejected the philosophical position that was called liberalism. And this vehement rejection has persisted through his followers. It's also anti-liberal in most broader senses.
> afaik Marx was and to some extent is (the ideas are from a long time ago) taken seriously as a serious theorist.
He's not taken seriously in philosophy or economics. There are some serious scholars who give him credit for contributions to early social science, although I think those scholars generally give him credit for ideas that were published and debated well before him and which he didn't really improve or advance.
Primarily he's taken seriously as an influential person in history, like Jesus, Buddha, Stalin, or Hitler. He was essentially a peripheral polemicist at the time of his death. The Soviets basically deified him because they needed something like a state religion, and they're responsible for whatever reputation he has today. But remember the Soviets were very big on propaganda, not so big on accuracy, and a lot of what people think they know about Marx is seen through the filter of authoritarian propaganda. Similarly a lot of what we know about Jesus passed through the filter of the Roman Empire and their desire to use him as the basis of an imperial religion. We're in a better shape with Marx, though, because we have many his writings (minus a significant amount of his correspondence which he and others destroyed).
There are, of course, economists and philosophers who consider themselves Marxists and who take him seriously. Often they're appointed to departments outside the main econ or philosophy department. For example, it wouldn't be unusual for a Marxist philosophy to be appointed to a German department or a history department.
