Kim is a totally different generation, and has completely different values to someone like Michelle Heaton. Michelle really had no basis for being so adamant in what she was saying, and she really did state it like it was a fact, not just her opinion.
Fair enough if she could back it up that she knew Kim personally, and could say that she was a totally different person off camera, however her total basis for accusing Kim of being 'fake', was essentially to use the term as an insult as she learned to do on her time on Big Brother. It's a discrediting tactic, when someone can't actually articulate a rational argument, or actually find a genuine fault with a person to just say they are being 'fake'.
After all, a rather matronly older lady, being on TV regularly, and (more famous, and liked, than Michelle herself!) for...cleaning... sour grapes anyone!