People's definitions of "singing skills" is just

First of all tone is NOT a skill, it isn't something a singer controls. The same basically applies to vocal range, which singers can only occasionally increase with regularly vocal coaching and practice.
The category of this should be breath control, pitch, diction and the strength of the notes.
For example, one of my faves is Marina, but I would probably rank her as a 6 at a push. She has an incredible range but sometimes she hits notes really badly live.
Someone like Mariah is a definite 10, in her prime her lower register was raspy, her pitch was absolutely flawless and her belts were clear and so incredibly controlled.
Please stop just ranking your fave as an amazingly skilled singer when they just aren't. This is about skill, not whether you like their voice or not.