What we'd culturally call "good" singing is about producing a loud, clear, consistent note. The best way to do so is to open your mouth wide with your tongue out of the way. When you do this the sound you make is "ah" (like when a dentist says say "ah")

Therefore "when the moon hits your eye like a big pizza pie" when sung is closer to "Wan tha moon hats yar eye lark a bag pazza pie".

As accents are mostly vowel sounds, it tends to wipe out accents in favour of musicality.

This is generally speaking, not applicable to every singer and every song and every word and every note.

Canadian.
Seriously, not the east coast Canadian 'aboot' but like southern Ontario is as neutral as it gets.