Not if they play Zekrom, it isn't; 6Corners is actually a very winnable matchup for The Thunderdome, because of the reactionary nature of the deck. Using Zekrom and Thundurus early on efficiently, and using the better speed, recovery and overall setup of The Thunderdome to full advantage, you can simply win by taking two Knockouts with your Basics and then Powering through with Magnezone from then on.
Magnetic Draw, Junk Arm and Catcher will win you the game, if you play smart and have a good list (and don't get donked by Leaf Wallop 0_0).
Just don't walk into Terrakion (Catcher it and kill it), and target down their energy if they don't have Terrakion yet (Catcher and KO whatever has the Energy), because after all, you don't want them dropping Shaymin to get you with a surprise Retaliate. The Strength of 6Corners is that it is very durable with Eviolites and uses Type Advantage against everything, but its weakness is that it is very difficult to keep on playing when your opponent just powers through your defenses. 6Corners just can't attach energy fast enough to recover from having its attacker quickly knocked out, and the Thunderdome can abuse that with impunity.