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In that case, no. Tiers are there for a reason. If Ubers were allowed you'd see every team sporting a Kyogre+Palkia duo, sweeping everyone. Having tiers allow battles to be more fair. However, you can do battles on the Uber tier, etc.
Actually, I got randomly challenged on Shoddy and the opponent DID send out a Mewtwo against me. I killed it and the rest of his team except for Magnezone because I got walled. And my last pokemon didn't have any attacks.