Yes, that Landorus is legit. "Takes plenty of siestas" is the correct translation of the Japanese characteristic ひるねをよくする, which was mistranslated to "Often dozes off" in Gens 4 and 5. However, they fixed it and made it "Takes plenty of siestas" in Gen 6.
The game corrects itself when it is transferred and its characteristic changes. This is completely normal, and the Pokemon's IVs do not change.