The 22-year-old singer said that she can understand why her fellow Scot was snubbed by the record industry and media professionals who voted on this year's shortlist.
Speaking to the Daily Record, MacDonald commented: "Susan has sold a phenomenal amount of albums - so you could ask, 'Why shouldn't she be nominated for a Brit?'
"But she hasn't changed anything musically or creatively and it wouldn't be a good advert for British music if what we thought deserved awards in the music industry was a woman singing covers."
MacDonald, who releases her second studio album in March, also suggested that Boyle's record label bosses do not have her best interests at heart.
She said: "I feel really bad for Susan because I think she has been exploited and the people behind her don't care about her. You can tell she is really fragile and being mobbed wherever she travels around the world. She has some problems and it is not right for someone like that to have this life thrust upon them."