The entire human civilization isn’t ruled by a single group/government, so it isn’t outlandish to compare different nations or nation-blocs.
But you do point to an interesting problem and that is one of humans’ impact to earth (and hence our habitat). Whether humans can adjust course without a single government coming up with rules and regulations remains to be seen.
Well, honestly it was more about the absolute scale metric: when a little portion of global population (5-10%) consumed more than 70% of the resources globally and was responsible for more than half of the global pollution over the past 3 generations, how bad is it in absolutes scale?
At least China represent 1/4 of global population and resemble a global government more than all western countries combined, including their power of influence over large scale events,like climate change.
If they bail out from climate talks and zero emission plans over, say, interference on the Taiwan issue, is it bad or good?
What is the best solution globally, pissing off China or human survival?
Yes, the USA is an aggressive and imperialist power with more than a few atrocities to its name.
However, this is not "how bad is China relative to the United States," but rather "how bad is China on an absolute scale."