There's definitely some people that are just Bad. I don't bother with more challenging content because I have no one to do it with and don't like doing stressful content with strangers, but pretty much every time I play MCH I have the highest DPS, except the one time I was in Dun Scaith when the highest DPS was a different (likely more experienced) MCH. In most content when you see stuff like that happen I reckon it's because everyone that has the potential to out-DPS the MCH is just not optimizing their rotations and stuff. I've out-DPSed people of all jobs but I think it's just because they were bad.