I don't think Marx would be powerful enough to somehow prevent the USSR altogether, or to have Lenin be cast aside. Those receiving his criticism would just dismiss him as an old man, still stuck in ideas from decades ago, not properly attuned to the modern day. This does not mean Marx wouldn't have any influence at all. I can see him criticising World War I as an evil war, fought at the behest of imperialists, aristocrats and capitalists-- to the detriment of the working classes of all nations. This might help, to some extent, in galvanising the socialists proclaiming just that after the war. As has been suggested in this thread, I can see Marx approving of German socialists like Luxemburg, while denouncing the USSR and the whole idea of an intellectual vanguard forcing a revolution upon a society whose masses are (in Marx's view) not actually ready for it themselves.
An effect of this would be that the whole "that was not real socialism!" excuse gains credibility, compared to OTL. On the other hand, if Marx's support for German socialists is sufficient enough to tip the scales and have their revolution succeed, then his reputation will be tied to the way that turns out. If 'Red Germany' is successful and largely works as Marx predicted, his view is legitimised. It proves, after all, that the excesses of the USSR were indeed due to the fact that russia was not yet fit for socialism. But if socialism becomes, ah, unpleasant in Germany as well, the result would be that the "that was not real socialism!" excuse instead loses its credibility. Because in that case, it'll be obvious that the failure wasn't due to a country "not being ready"-- which implies the failure is instead inherent to socialism.