Cowell made the suggestion during filming backstage for the ITV2 series, advising Mulhern that he needed work done on his hair, nose and face.
An insider show source said: "Simon [Cowell] told [Stephen] that he needs improvement. He said that he needs a hair transplant, a nose job and some work on his chin - perhaps he's trying to mould him into a mini me?"
Mulhern's "worst ever day filming" on Britain's Got More Talent was rounded off by a trip to hospital to treat a mystery illness.
"Filming on More Talent was halted for three hours as Stephen had to be rushed to an emergency doctor as his throat swelled and he lost his voice," the source told Digital Spy.
"He returned to filming after doctors were able to treat his mysterious and sudden illness with antibiotics."
Britain's Got Talent's Cardiff auditions were filmed at the weekend with judges Cowell, David Walliams and Alesha Dixon. The auditions continue on February 6 at London's Apollo, before heading to Edinburgh and Birmingham.
> 'Britain's Got Talent' Alesha Dixon misses partner in crime Holden
> Simon Cowell 'Got Talent' will get 20m viewers back in a heartbeat
Watch Ant & Dec talking about the 2012 Britain's Got Talent auditions: