This is not a knock on what she has already accomplished, or on American Pharoah. There is no doubt that he was an extraordinary racehorse. Nonetheless, he is only one horse, and there is no guarantee that Littleprincessemma will produce anything else that can get up a good trot. Her odds of getting another good runner are certainly better than those of most other mares, given the caliber of stallions she has been visiting and the likelihood that her foals will be in the care of elite trainers, but my point is this: she hasn't done it yet.
Littleprincessemma is not the first "one-hit wonder" to make the list of Kentucky Broodmares of the Year: Maid of Flight (1964), Kerala (1967) and All Beautiful (1969) all received similar honors based on the exploits of their great sons Kelso, Damascus and Arts and Letters. Like Littleprincessemma, all were relatively young mares who could reasonably be expected to add further laurels to their records---and all failed to do so.
On the other side of the coin, the production of a Triple Crown winner has not always been a ticket to a Broodmare of the Year title. Five Triple Crown winners won their diadems in years in which their accomplishments could be considered as a factor in the choice of a Broodmare of the Year award. Of those, only Somethingroyal, dam of Secretariat, gained the title, and she had already produced the major winners Sir Gaylord, First Family and Syrian Sea before Secretariat put the crowning touch on her production record. The other four? Well, Won't Tell You, the dam of Affirmed, wasn't eligible as she was based in Florida, and neither was Assault's dam Igual, whose home was the King Ranch in Texas. The others were Hydroplane II, dam of the immortal Citation, and My Charmer, who produced the English Classic winner Lomond and Group II winner Seattle Dancer as well as Seattle Slew.
Of course, the obvious question is that if Littleprincessemma had not been named Broodmare of the Year, who else could have been? My own choice might have fallen on Clarkland Farm's Leslie's Lady, whose daughter Beholder accomplished a triple of her own last year by winning her third Eclipse Award from four seasons of racing. That's a rare feat in itself, and Leslie's Lady is also the dam of Grade I winner and successful sire Into Mischief. Others may have different opinions. As for Littleprincessemma, only time will tell whether she will become one of the great matrons of the breed or whether American Pharoah will remain the sole jewel in her crown.