I'm not expert, but is this something actually could happen?
I assume that if two dumb people mate rather than all of their offspring being even dumber there is at least an equal chance of the offspring being brighter than the parents?
It was a completely fictional comedy movie, not a documentary.
To answer your question though, anything could happen. However, the premise of the movie isn't very sound (again, it is fiction). You are correct. A child can be brighter than both parents.
I assume that if two dumb people mate rather than all of their offspring being even dumber there is at least an equal chance of the offspring being brighter than the parents?