I should have clarified - it's idiotic in the context of a group that is still clearing the fight with few, if any, incidents.
In the case of failure, I agree - it's important to look at causation, but that includes all factors, not just "someone's DPS is low".
tl;dr OP shouldn't be berated for doing "slightly" lower DPS while handling mechanics but should work on multi-tasking if they are doing "significantly" lower DPS and using "but mechanics!" as a blanket excuse.