This is one of those questions where I wish I had a solid answer. Cooked meat is not much different than raw, in terms of basic nutrients. Obviously, you wouldn't want to feed an axolotl meat that has been salted, seasoned, fried, etc, as this would change the composition.
Somehow, cooked just seems odd to me. Axolotls or other sals might eat some pure muscle tissue in the wild (fish, etc.), but it would never be cooked. So my gut feeling is that cooked meat is all wrong, but I don't have any real evidence to back that up.