Nope. Nothing can fully counter a Mewtwo; some can make it think twice, though. Chansey is probably the best counter. Light Screen Chansey, more specifically, can stall it and aim for a freeze. However, Mewtwo has a high critical hit rate and Amnesia on its side. What's more, it could backfire on you; Mewtwo can set up easily against Chansey, and will get to 999 Special after two Amnesia boosts. If it gets past Chansey, you can say goodbye to your whole team. Mewtwo will also be aiming for the freeze on your Chansey with Ice Beam / Blizzard, or just spamming Psychic for a Special fall
The Psychic / Blizzard model is stopped cold, as far as resistances go, by Slowbro, and, to an extent, Starmie, but they can't touch Mewtwo either. Also, watch out for Snorlax with Self-Destruct
If you ever face a Mewtwo, paralyze it at all costs, unless you have a Mewtwo, Slowbro, or Chansey of your own, and want to play dice with their Mewtwo by going for a freeze. This cannot be stressed enough--a Mewtwo that is not paralyzed will tear apart your whole team. You CAN stop a paralyzed one with some luck and one sacrifice if your opponent is stupid enough not to switch out. A fresh Mewtwo will destroy every single one of your Pokemon after setting up, and you won't be able to do anything about it. And, well, if Mewtwo has Rest and your opponent is skilled... good luck
>Physical blows are where Mewtwo is most vulnerable (although it's not weak on that side, with defensive capabilities about halfway between Tauros and Slowbro), but with an Amnesia, Mewtwo can devastate all of the physical attackers that it is weak to
According to that, you cannot fully counter a Mewtwo. But tldr, Chansey can stall it out, psychic blizzard ones are stopped by Slowbro and Starmie but can't touch it, paralysis, Snorlax boom
Source: https://www.smogon.com/dex/rb/pokemon/mewtwo/
