The Magic Mirror is pure fantasy and it is excellent. Somewhat comparable to "My Friend, Mr. Nobody". This is a Penny feature.
Penny and Dr. Smith discover a platinum magic mirror. Of course, Dr. Smith's greed kicks in. Debbie The Bloop and eventually Penny and Dr. Smith enter the mirror, which only animals can exit.
Yes, the props, once inside the other dimension, are questionable, but this is a fantasy/ dream-like setting, so it works just fine. Michael J. Pollard plays the boy inside the dimension. Dr. Smith, attempting to explain the situation, while within the dimension is also a good fit. That said, he could have had similar thoughts in other episodes, but remained mum.
The story is excellent and it is always a treat to have Penny (and Debbie The Bloop) and the center of the story.