This is a bit like saying earnings going up isn't good for stockholders, because they would have been charged a higher price to buy the stock if people had known the earnings were going to go up.

Once you've taken out a fixed-rate mortgage, inflation absolutely has the effect of reducing the value of the debt you owe. It's more if you're about to take out a mortgage that you're rooting against (expectations of) inflation, as lower inflation will also serve to decrease the prevailing interest rate.