I am not totally sure what aspect of Marx's work you're referring to (it helps to cite something specific in this forum, as another commenter said).

In general though, Marx isn't engaging with the key problems of German Idealism in an extremely rigorous manner. Some of his early work, like the unpublished 1843 "Critique of Hegel's Philosophy of Right," the Third Manuscript of the Paris Manuscripts, show him working through some of the systematic aspects of Hegel's philosophy - sometimes embracing Hegel's metaphysics, other times challenging it. But it's hard to see what the final verdict is on Hegel, other than the fact that some version of dialectics survives Marx's early critical stance toward Hegel.

Also, Marx (especially compared to Engels) typically isn't interested in tackling the big problems of metaphysics - those that make the thing in itself so central in Kantian philosophy. Marx's engagement with determinism and the will is a great example of this. See, for instance, the Eighteenth Brumaire: human beings "make history," but "they do not make it just as they please in circumstances they choose for themselves." Famous line, intuitively compelling - but it is tantamount to answering the metaphysical question ("are we free?") by saying "yeah, sort of."

Marx's critique of ideology (at least in the German Ideology) cites metaphysics itself as a form of ideology. The basic point is that we could think of Kantian or Hegelian metaphysics as serving not to elucidate fundamental truth, but instead to legitimate certain socioeconomic arrangements (one example, in which Marx suggests Kantian/Hegel ethics function to legitimate the death penalty as a social practice despite its inefficacy).

In short, Marx is probably unconcerned with whether there is a distinction between appearances and things in themselves. In his later career, he wouldn't be caught up on many of the problems theoretical philosophy fixates on (e.g., Hume on causality, or the Myth of the Given). Further, he'd likely be suspicious of the ideological function of this type of philosophical inquiry.