Hilary Mantel states it as fact.
If Reginald ever truly did harbor negative feelings toward his mother, he had certainly overcome them long before her death. He also was not formally dedicated to the church until he did so under his own power as an adult. In truth, Margaret had sent Reginald to be educated at the king's expense, both of them recognizing his unusual aptitude for learning. Long after he was out from under the rule of his mother, Reginald became a Cardinal, though he would not become an ordained priest until 1556, long after Margaret's death.
Having clarified that, Reginald was grief-stricken by the death of his mother, which closely followed the execution of his oldest brother, Henry Pole Baron Montague. Reginald was in Capranica, having recently left Regensburg, when he received the news. (The medieval center of Regensburg is now a UNESCO World Heritage Site. Now in Germany, during Pole's time it was in the Duchy of Bavaria.) He had been there as part of a conference to discuss the Biblical issue of justification, a failed attempt based upon the city's conversion to Lutheranism by the following year. Pole was one of the few legates looking for compromise, a habit that would lead one contemporary to say, "He has been very unfortunate . . . being considered a Lutheran in Rome, in Germany a papist."
Until now I have believed that the lord God has given me the grace to be the son of one of the best and most honored ladies of England and I have gloried in that and given thanks to His Divine Majesty. But he has wished to honor me more and increase my obligation, for he has also made me the son of a martyr, whom that king, because she was constant in the Catholic faith, has had publicly decapitated, even though she was more than seventy years old (she was actually 67) and his aunt. Thus he (King Henry VIII) has rewarded the efforts which she took for a long time in raising his daughter (Princess Mary to whom Margaret was governess). God be praised and thanked.
To the abundant letters of condolence received by Pole he would reply that they were blessed to have one more advocate in heaven. Reginald was a man of staunch faith and controlled emotions, but he could not completely hide his sadness at the harsh treatment of his mother.
Some of this may have been induced by guilt. One of the reasons the Pole family in England was targeted by Henry VIII was due to their refusal to abandon the Catholic faith in favor of Henry's new Church of England. Reginald had written to Henry many times criticizing his actions and begging him to return to God. Henry, never one to take criticism well, sent assassins after Reginald. Failing this, he turned to Reginald's family. It was not difficult to do something that could be construed as treason in Henry's England, but, in Margaret's case, she was never charged or given a trial during her two years of imprisonment before her execution.
Even Pope Paul III recognized the depth of Reginald's grief, giving him extra time away from Rome and saying that he would spare him discussion of the matter. Reginald wrote to a woman named Colonna that he looked to her now as a mother and that none had consoled him better than she. He also spent much of the summer of 1541 closeted with Psalms in study and meditation.
Reginald Pole spent the remainder of his life striving for reconciliation between the Catholic Church and the reformers. His position as a Cardinal made him untrustworthy to Lutherans, while his willingness to listen to opposition and concede that some of their arguments made sense put him under investigation by the Inquisition. The last years of his life were spent with Queen Mary I in England, assisting with her attempt at counter-reformation there. He died, a Cardinal of the Catholic Church and Archbishop of Canterbury of the Church of England, on November 17, 1558, the same day as Queen Mary.
For more on Reginald Pole, I recommend Thomas F Mayer's Reginald Pole: Prince & Prophet.