Simon Cowell has clearly had extensive plastic surgery because his face just isn't looking natural on certain days. He has the appearance of having had facial fillers, a facelift, and Botox. He also shows signs of "shiny face" and bloated looking cheeks and it's not going unnoticed by other celebrities. David Walliams recently poked fun at Simon Cowell's overdone plastic surgery as well as his age (which was in bad taste)! Simon Cowell obviously didn't take appreciate the remarks, but he moved on. Just take a look at how shiny and puffy his face is.
Simon Cowell has publicly said that he's not at all opposed to plastic surgery. Let's be honest, he has public image that he has to keep up. He was the judge on American Idol and is currently the judge on The X-Factor. Also, he has countless public appearances where picture after picture is taken. So, if Simon Cowell needs a little plastic surgery to keep up his appearance then there's nothing wrong with it.
Sometimes you just have an off day... he looks better in most other pictures.