Yeah Whataboutism is typically used to diminish the severity of the original issue being responded to.

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.