Kelly Clarkson is a good singer. She simply sings songs that don't interest me. Conversely, Kai is a bad singer, who sings music which if performed with a talented singer, would interest me.
As for your fact vs. opinion argument, you must first qualify it with what is meant by being able to sing. If all you require, to be able to state that someone "can sing", is that they possess the ability to stay in key, than yes, Kai can sing. However, Kai lacks power, lacks range, lacks nuance, and lacks the ability to convey emotion. To me, that means he can't sing. As an example, if you asked Kai to sing Lance's vocal lines from one of the Pyramaze CD, it would sound like a heard of cats sustaining simultaneous blunt force trauma.
Zod