An 'ultimate mash-up' of stars who found fame on the ITV show was released last month ahead of its tenth run.
One Direction, Leona Lewis, Alexandra Burke, JLS, Cher Lloyd and Little Mix are among those who were included in the clip, but Cardle was snubbed.
Melanie, whose duet with Cardle is out now, told the Sunday People: "I think it's really f**king rude the way they have left him out.
"He was such a great winner and he's gone on to do brilliant things. Matt's possibly one of the best winners on the show.
"With the musicality and the talent, he has longevity ahead. And let's face it, that doesn't fall into the life of many X Factor winners.
"Matt knows what he wants musically in the studio, he writes and produces and it stands him in good stead in the industry for the future."
Cardle said that he "expected to be cut from the mash-up video", while his manager Will Talbot told Digital Spy that it is "disappointing to see Syco attempting to erase Matt from the history books".
Melanie recently revealed that she thinks Louis Walsh exiting The X Factor will be beneficial, asking: "What does he actually do?"
Watch Matt Cardle and Melanie C in the video for 'Loving You' below: