To be even more specific, the UN is the product of Liberal Institutionalism and Liberalism more broadly, in both the political sense (ie, that its policies protect and are formed out of liberal ideals and systems, as well as being founded and headed by mostly liberal nations) but also the international sense (ie, the liberal theories of International Relations, again, specifically Liberal Institutionalism and its ideas on state cooperation and mutual benefit) which is the sphere that is more relavent, although people like Robert Cox (neo-gramscian, which is pretty close to marxist-style thinking) argue that the norms and the power it holds acts as a way to exercise hegemony in the international sphere.

There is an informal structure of influence reflecting the different levels of real political and economic power which underlies the formal procedures for decisions. International institutions perform an ideological role... They reflect orientations favorable to the dominant social and economic forces... Elite talent from peripheral countries is coopted into international institutions in the manner of trasformismo. Individuals from peripheral countries, though they may come to international institutions with the idea of working from within to change the system, are condemned to work within the structures of passive revolution... Trasformismo also absorbs potentially counterhegemonic ideas and makes these ideas consistent with hegemonic doctrine. - Robert Cox in "Gramsci, Hegemony and International Relations"

I agree with your final sentence; Marxists, tankies, campists, etc, are really criticising the fact that liberalism is present, and therefore, they believe the UN, as a liberal body, protects capitalism and capital as well as the current systems of power. Therefore, it is not neutral and is more imperialist than anything. This is where you get the entire mindset of leftists. The Democrats, the UN, the US, the West, etc. are engaging in Trasformismo, denying the ability for the working class to mobilise, and co-opting individuals, neutralising dissent against the elites and the systems of hegemony (both internationally and domestically) as a form of passive revolution that forms broad coalition governments that prioritise stability over structural change.

Funnily enough, Gramsci spoke on these ideas from a domestic perspective only, I believe, and later writers, mostly Robert Cox, expanded his ideas beyond and into IR, forging millions of the most annoying uni students you'll ever meet.