Yet there are gurus, sadhus, and sanyasis (men devoted to holiness) in India who regularly smoke as part of their practice. They are devoted to the god Shiva and it is part of their ritual, which includes extensive meditation.
Meditation, under any circumstance, is better than no meditation at all. The question is, how effective would meditation be if you smoke weed before it? If you just nod off and go to sleep, then it would not be an effective way to go about meditation.