Here's the banlist for Haxmons:
King's Rock: The fact that it lets every move flinch is too strong, it's nearly unstoppable when paired with priority.
Taken from this forum. As you can see, the only Ability that is banned is Stench. So the answer is no, Sniper is not banned. Haxmons is more of a just-for-fun metagame and only aspects that would make it less fun are removed. Sniper is simply a damage increase.