The issue associated with whataboutism is that it is a deflection strategy. It aims to steer the focus away from the issue at hand. Commonly regarded as intentional manipulation or lack of adherence to social conversation norms.
I'm not sure if that's exactly the right word here, i.e. in my example earlier I'm obviously not defending lakes of toxic sludge. I just thought it was remarkable that someone claimed a lake of sludge was the most morally devastating thing they'd ever encountered, on this planet, where the mere act of making a sandwich typically involves a far more horrifying causal chain (e.g. the animals lived in darkness, in a cage so small they couldn't even turn around).
Maybe there's a better word for that? Putting things into perspective? I don't know.
Focus (on a society-wide level) is not so very limited. One of the strengths of our whole society being fragmented into a bunch of distinct organisations, is that they can all go off and solve different problems at the same time.
> Shouldn’t problems be addressed in order of importance?
Most of the really big problems facing society can only be addressed through multi-solving (i.e. something like climate change isn't going to just hang around waiting for us to solve one sub-problem at a time). Not to mention, the solution space is often extremely interconnected - for example, finding alternatives so that society can eat less meat helps address both climate change and the horrors of factory farming at the same time.
Whataboutism would be "why do you worry about your wrecked car, that crackhead down the street also has a wrecked car".