Cannon means included as part. You'll see this a lot in anime/video game debates as well as in debates about the bible. If it's cannon, then it means it's included as part of the series. If it's not, then it's just a side story holding little or no value to the topic at hand.
If you replay Majora's Mask you'll find that the Happy Mask Salesman talks about it in the beginning. It was used by a tribe for hexing rituals and such.
Assuming the "wish armor" thing is true, then it's possible the armor or its creator had evil intent and were going to use it to cause harm...