I think you've got one more "don't" in there than you intended. You believe people Do recognize the exceptional quality of the singing - but still don't appreciate or enjoy it.

The reason I put quotes around "don't care for" is that I think those who don't appreciate opera, don't "get" opera. It is the highest form of performing art; it isn't for anyone to dismiss, wholesale. A person who will open their ears and their heart will learn to appreciate opera.

Anyway, my main point is that good and not-so-good singing is an objectively determinable thing - to someone who knows how to listen.