Yupp! Baron Mordo is already aware of what mistake Strange just made when Strange was using his newly learnt trick to enclose all(people in the scene) in a mirror dimension.
Strange thought about civilians and enclosed all in the mirror dimension to leave the real-world unaffected of the following fight.
If you notice Baron Mordo's expressions just after Strange uses the mirror dimension, you will realize that Mordo realizes quick that Dark magicians are stronger in the mirror dimension and they can get killed herein.
How Mordo knows this is not disclosed in the movie however, maybe how Strange learnt this, via practical implementation I guess.