The 20-year-old former Disney star appeared on Jimmy Kimmel Live, and aimed some tongue-in-cheek barbs at Cowell's image.
Speaking about taking the chair next to Cowell on the talent competition panel, the singer admitted it was hard not to look down his top as "he shows more cleavage than I do".
When asked whether she would ever date the 53-year-old if he was 25, Lovato said no. When Kimmel asked why, Lovato joked: "He parts his hair down the middle."
[Demi Lovato with Nick Jonas at Judges' Houses on 'The X Factor' USA]
Kimmel also quizzed Lovato on why she had been moved away from fellow judge Britney Spears and next to Cowell, but she dismissed it, saying: "We were just playing musical chairs. They hated me being over to shout at Simon, so they were like, 'We'll just put you next to him'."
When asked about Spears's reaction, Lovato said she wasn't consulted, before joking: "Things just happen over there, there's no professional courtesy at all."
Lovato was recently reported to be "loosely dating" One Direction's Niall Horan, but was quoted as saying that her commitment to The X Factor USA means that it is difficult to have a proper relationship.
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