Sure, it might be reducing complexity, but it might not be. Consider the Library of Babel example, and bear in mind that a brain-state has a ton of extra information over and above the 'mental state' it supports. (Though strictly speaking this depends on the notion of 'mental state', which is indeterminate.)

Also, we have to ask "reducing complexity relative to what?" (As I said above, there are many possibilities other than "literal description" and "our universe + pointer".)