St. Malachi died in the year 1148. The so-called prophecy was "discovered" in 1595. Significantly, the "prophecies" from the intervening 447 years (1148-1595) were a pretty good match (as they would be, if they were written after the fact!!) and the "prophecies" dealing with popes after 1595 are ludicrously vague, very much like the prophecies in fortune cookies.
They can all be nicely interpreted ex post facto, of course.
But the Church's attitude is that it's not private revelation, it's a forgery.