Is chanting Nam Myo Ho Renge Kyo positive thinking?

Chanting is neither meditation, nor positive thinking, though it reaps the benefits of both these practices and much more. The essence of chanting Nam-myoho-renge-kyo is that in the very act of doing so we are expressing our Buddha nature. Meditation and positive thinking do not have Buddhahood as their objective. It is not possible to express Buddhahood through these means. Although meditation and positive thinking may have value they cannot change the fundamental element in our lives which makes us unhappy and unfulfilled as human beings —our karma, nor can they bring out the highest condition of life, Buddhahood.

Meditation is a more passive exercise than chanting; one usually calms one's mind by concentrating on a particular phrase or image. At first glance this may seem close to the practice of Nichiren Daishonin's Buddhism, but actually the difference is apparent. The practice of chanting Nam-myoho-renge-kyo enables us to express and experience our innate Buddhahood and release the powerful energy contained within that, rather than to calm our minds or negate certain ways of thinking.
Again, while it is true that our thinking does become more positive as a result of chanting, this is because chanting draws out our Buddhahood which, in turn, influences every aspect of our lives, both mental and physical. Therefore, chanting is not so much a question of "thinking positively" or exercising "mind over matter," which implies restriction; rather, through chanting, our highest state of life naturally influences our thoughts and actions towards the most valuable ends.