It most certainly is. A literate person knows the "AD" is placed in front of the date. If one wants to place the era indicator after the date, it is suggested one use "CE," for "Common Era" and "BCE," for "Before Common Era." If one insists upon using "AD" and "BC" it is incumbent to know the proper uses.
Sloppy writing is indicative of sloppy thinking. If the writer is too lazy or ignorant to get something as simple as the placement of "AD" right, why should I trust him on anything else?