Is Marxism common among anarchism

Its difficult to say exactly, its not as if there is a handy ready made sample of the worlds anarchists for us to poll.

I do see that many of the users here are anarcho-communists and such but want to have a better grasp on where the community lands when it comes to Marx specifically.

Interestingly notably anarcho communists of history (Kropotkin, Malatesta, Goldman) were notably not Marxists. The first person generally considered to be the forebare of what we now call anarcho communism in the modern day, Dejaque, predated most of Marx's mature work. Honestly, reading anarchist literature from the past i find it curious that there seems to be a notable ammount of anarchists in spaces like this who are in anyway positive of him. I suspect that probably due to marx's popularity in the broarder sphere of leftism Marx's critique of capital brings in a lot of people who then go for an antiauthoritarian route. which brings us to your next point.

More to the point, is Marx’s theory of value popular among supporters of Anarchism and is it a popular justification for ending capitalism?

Marx is superfluous, many thinkers have put forward critiques of capitalism. Marx's value theory is not his own, he derived it from the ricardian socialists and the liberal david ricardo. Capitalism was seen as exploitative from years before Marx wrote capital, oppressive from before he wrote the manifesto, and the mechanism of that exploitation even was idenitifed by, among others, the liberals (they called it profit) The falling rate of profit (often cited as why capitalism is mechanistically predisposed to collapse) was similarly identified before Marx was even in school let allone university as an empyrical phenomenon by Smith.

In general capitalism is predicated on there being a gulf between the value of a product obtained by the capitalist from labour and the wage that the capitalist pays that labour. This isn't a marxist thing, this is a capitalist thing, its litterly what profit is. We can criticise that as exploitative without a specific idea of value, if capitalism is functioning then no matter what economic outlook is correct or in opperation is exploitative. We can further go on to point out that such exploitation is simply another instance of the goivernment of man by man, oppression by its very nature, which is why anarchists oppose capitalism.

Is Marx popular? Maybe. Is he neccessary? No.