I agree about not all sims singing, or at least they should not sing in the same way.
Also, I wish the singing features would be better synced with singing skill. A sim with no singing skills do sing the holiday cheers nearly perfectly, but if starting to practice singing it sounds terrible (as feels right on the first levels). It would be better if both singing holiday cheers and singing in the shower would reflect the singing skill level. We should at least have 2 versions (bad or good), different singing quality would not be needed for every single singing skill level. Just don't want my total newbie to sing the same way as the skilled superstar.