Kpop fans are not rap fans, we hold different standards because you know, those are different entertainments. If you're a rap fan it's ok to have the same standard regardless if it's western rapper on an idol rapper, in the end you like what you like
However don't expect people who likes good vocalists to not think the same of most of kpop idols, because most of them can't sing. And Mariah Carey level if far from what most kpop fans complain, most of us just want idols who can't hit their notes without being out of tune and voice cracking 80% of time. Nowadays kpop idols are autotuning and lipsyching even ballads
And yes, there is a clear objective part when analyzing vocalists. Singing style aside, you can easily say if someone is out of the key or if they are strain and you don't need to be a vocal coach for that, you only need a ear
So, no. It's not a different subject, it's exactly the same subject just judged by different standards