Can Apple Butter be substituted for Apple Sauce in baking?
You start with apples, cook them a bit and you have apple sauce.  Cook that sauce very slowly but for a long time and you get apple butter.

While I think the substitution would generally work, it most likely will be  sweeter. Sweeter because there's less water in apple butter vs. apple sauce thus concentrating the sugars.  Also the brown to dark brown color of apple butter is caramelization of the apple sugar.  That caramelization has to change the taste of your muffins.   

Give it a try and see what happens.  Then make another batch with less sugar and see how they turn out.  If your recipe uses brown sugar try changing some or all for white sugar using the apple butter as a kind of substitute.  I suspect somewhere in those tests you'll find a batch that's great!