Campism is more or less the idea that there are imperialist states (namely the West led by the US) and anti-imperialist states (the global periphery/third world/etc.) and if you don't support one "camp" you have to support the other. That's maybe a bit reductive but campism is a bit like "tankie" in that nobody seriously calls themselves a "campist" and it's used as a derogatory term so I won't dive into it too much.

I'm not sure what multipolarism is but I'm assuming it's supporting a multipolar world order where The US, Russia, China, and maybe India are the poles by helping to shrink US hegemony and grow the other poles'. Depending on the flavor of Maoism it might be a little unusual to levy this critique, but it certainly comes out of the ultra-left camp. The argument would boil down to it's pointless if not actively harmful for the left to support Russia, China, etc. because these are not socialist states (according to Maoists) and the working class is well aware of this. These are imperialist powers in their own right and supporting these bourgeois national wars is detrimental to the international proletariat and the revolution.