Is it completely useless? No. If you can get Mewtwo Level X down early, you can potentially have an easy 2-3 turns in which direct attacks aren't going to hurt you. Add the fact that the best G attacker is weak to Psychic, and you have the potential to earn a rushing win.
That said, Skuntank can still poison you, Crobat can still ping you, Honchkrow can snipe your bench, and eventually a good G deck will get that Dialga out. Also, Energy Gain limits the benefit of Mewtwo(LA)'s Psychic attack, and your weakness to Psychic means that, once the Dialga is down, any remaining Toxicroak will quickly finish you off.
On the whole, a well-built Mewtwo deck (like any other well-built deck that can do 90 damage to Toxicroak) have the potential to give a G deck a run for its money. But, don't expect the battle to be easy, because you'll probably find that you're the one with a quick loss, if you do.