well they already call him Mats, so you lose that battle.
Let the players call him Mats. Virtually none of them played with Sundin, so it's not disrespectful. However, I don't think fans should call him Mats. Most of the Leafs fanbase still remembers Mats Sundin, so it feels disrespectful for them to use that nickname.
Fans using players' nicknames for each other also feels like the same sort of crap as fans saying "we" -- like they're deluding themselves into thinking they're part of the team.