A lot of people consider marijuana and DMT to be a bad mix. THC has been known to cause anxiety, and when you add that to the intensity of the DMT experience, it can be a brief jaunt into utter Hell.
Personally, I find the euphoria associated with the come-down is a fine time to be high on marijuana, although I really don't enjoy the feeling, so I don't do it all that often.
Blessings
~ND