I don't believe it is.

1. Shakespeare's use of supernatual and magical forces in this play would have appealed to an Elizabethan audience. They wanted to believe in these types of things in addition to fate. Saying that these magical things happened only because it was a dream would have possibly been an insult to the intelligence of the groundlings in attendance.

2. The plot involves too much stuff to be a dream. There are essentially four different plot lines all intertwined into this play. Even the most complicated of dreams really takes that width.

3. Most importantly, Puck asks up to think of it as a dream if we didn't like it. When was the last time you had a dream where one of your main characters told you to pretend it was a dream? Doesn't happen. The very premise of dreams is that our sleeping perception makes them reality. Had MSND actually been a dream, no character would have called attention to it.

Hope that helps. I'm sure there are some others out there that can give you other reasons as well!