The comic reportedly made fun of Simon Cowell at the Manchester auditions by calling him camp and mocking his plastic surgery.
After Cowell told an act to be more "showbizzy", Walliams joked: "Yeah, have a bit of work done, become friends with Louis Walsh, go on holiday with Sinitta, have your picture taken with your top off and some fruit in your mouth," referring to recent tabloid photos.
"Simon picked David because he likes his sense of humour and knew he'd be a success. But in doing so he may just have put himself in the shade - and that's somewhere he doesn't enjoy being," a source told the Sunday Mail.
"He has never had someone take the mickey out of him so mercilessly, especially on his own show. And although Simon came out with a few classic moments, David stole the show.
"Some of the jokes were very personal and there were a couple of occasions when Simon looked uncomfortable. But all in all it was taken in good spirits. No one else on the planet could have got away with saying all of that."
Walliams made his debut alongside Cowell, fellow newcomer Alesha Dixon and veteran judge Amanda Holden.
Watch the stars of Britain's Got Talent talk to Digital Spy below: