The looks attributed to by-pass surgery has much to do with the diet and medications you are on that control unhealthy eating habits and the effects those foods have on you.
Many of the foods Clinton was eating prior to the surgery gave him a 'meaty' robust appearance and it was deceiving. He has said many times that he had stopped taking his meds, too, for years prior to the surgery which also hid the fact that the foods that were clogging up his arteries created an 'out of control' situation. In many cases, the rapid/immediate loss of 'bad' foods/weight gain in combination with taking the meds on a regular basis to keep things under control, give patients a rapid look of aging, even though they are actually healthier internally than they have been for a long time.
Clinton has lost quite a bit of weight since his surgery and it has made him appear to have aged rapidly because of the 'sunken' look to his face instead of that 'pudgy, healthy glow' that was a false appearance of good health before. I've seen this happen with quite a few heart surgery patients over the last ten years, including my mom....as well as some people I know who are being treated for diabetes in their later years now. Adjusting to a new regime that needs to be followed immediately gives an extreme appearance change more often than people realize, whereas the change would have been a gradual one otherwise and nobody would have noticed it so much.
I'm watching footage of President's Bush 41 and Clinton in Tailand right now. Bush is 20+ years older than Clinton, yet they look far closer in age. The recent heart problems that Clinton has had no-doubt have aged him tremendously. I hope he bounces back a bit better in the years to come. He just doesn't look so good.
(p.s. This is NOT a Clinton bashing post. I was just a bit shocked by his appearance post bypass surgery. My FIL was around 65 when he went through this and he looked much better than before the surgery within a matter of a few months)