Kara DioGuardi says replacing Simon Cowell on 'American Idol' is going to be difficult.

The judge and songwriter admits there are few people who will be able to fill Simon's shoes when he leaves at the end of the current season, but she thinks record boss Tommy Mottola could be a good choice.

She said: "Finding someone who has a lot of knowledge about music and who is also an interesting character is going to be a daunting task.

"I like Tommy Mottola, because he has launched the careers of a lot of superstars. but whoever gets the call has got to be a character."

She also admits she sometimes finds it difficult to believe she is on 'American Idol' with such big personalities - chat show host Ellen Degeneres, record producer Randy Jackson and host Ryan Seacrest.

She said: "Could I be next to bigger people? My God - Ryan Seacrest, Ellen, Simon and Randy! Sometimes I look in the hallway and think, 'There's a billion dollars in this building!' "

Kara - who has written songs for Pink, Ashlee Simpson and Gwen Stefani in the past - claims some of the viewers of the show are particularly critical about her judging ability.

She told Women's Health Magazine: "People are so mean. they'll go, 'Kara's stupid. She repeats whatever Randy says. Paula's way better' or 'she's ugly'. I'm like, really? I may not be some beauty but ugly.

Simon is leaving 'American Idol' to begin a US version of his British talent show 'The X Factor' - it has not yet been confirmed who will be on the judging panel for his new venture, although Girls Aloud singer Cheryl Cole is thought to be lined up for a role.