It is a huge gray area, for sure, but I think there is a difference between "better" and "preference". However, people generally do use "better" to mean they prefer it more. Perhaps because music was such a huge part of my life when I was younger, and that meant I dealt a lot with music and artists that were not my preference, I developed a bit of perspective that people who are pure music fans would not have.
To be specific, I never had a musical preference for Michael Jackson. But he was clearly great at what he did, broke new ground in videos and musically, crossed ethnic lines and deserves to be known as a great artist. Doesn't mean I prefer his style of music.
Today, Taylor Swift polarizes people musically, but she is without doubt a creative genius and one of the best concert artists out there today. She is great at what she does, and someone can be acknowledged while also saying they don't prefer her type of music.
On the opposite side, I really like Skillet. My son and I have seen many of their concerts. But if I had to write a list of the "best" artists out there, they would not be top of the list. Still, when speaking generally (to take goingbannanas point) I would say "Skillet is totally better than Michael Jackson" which I think would be understood as my preference and not a statement on their artistic ability and influence.