Princess Mary was far from satisfied with her father's new wife. The dislike may have routes in two things: first of all, Katherine was 9 years younger than Mary and it is possible that Mary may have felt jealous at this young woman who was certainly more unintelligent than herself but still married - it was considered an embarrassment to grow old as a spinster and despite that Mary was not old, she was still older than her father's new wife. Perhaps Katherine reminded Mary of what she wanted - a household of her own.
Secondly, Katherine's silly behaviour was so far from what Mary had experienced from her first years with her mother, Catherine of Aragon, that Mary may have found it difficult to accept a young, giggling girl sitting at the throne her mother had sat in. It is very likely that the immensely proud Mary saw the marriage of her father as a demeaning to the image of an ever-dignified Queen that Catherine of Aragon had imprinted on her daughter.
But the dislike turned into a scandal when Katherine noticed the hostility from Princess Mary and consequently complained to her husband. To prove her new status Katherine had two of Princess Mary's maids removed which was a public humiliation and the scandal was well-known.