The Mind Equals Reality Principle is a very general philosophical principle of the CTMU, and despite its seemingly important sounding name, is not particularly controversial at all. M = R is essentially just the principle that inasmuch as reality exists, it conforms to certain laws or categories of cognition and perception. In other words, the universe exists in such a way that we can apprehend its existence through sensory perception, including the five sensory modalities. This is pretty much just a restatement of the age-old thesis of empiricism - that we can know the world through the senses, and in that sense, it may be the most common-sense and simplistic of the CTMU principles.
The M=R principle does not state that you can simply think something into existence, or that the world doesn't exist when you're not looking at it, or anything ridiculous like that. It simply states that the universe possesses general laws that conform to the categories and laws of cognition and perception (i.e. sensory perception and thought).
M=R asserts that mind and reality are ultimately inseparable to the extent that they share common rules of structure and processing.