Not really. Salamence is generally best with Return / Dragon Claw / Dragon Dance / Roost or Substitute. Thrash, while it would be handy when hanging on at 3% after an Ice Beam, would have to take away either MegaMence's setup, recovery, damage shield, or Dragon STAB. It is not a reliable replacement for Return or even Double-Edge due to it being just a Normal (Flying)-type Outrage – it's really powerful if the conditions are right, but it also leaves Salamence vulnereable to speedy Fairies and Ice-types. In short, the reliability of Return is better than the riskiness of Thrash. That, and they are the exact same base power.