I don't want to sound negative, but I'm not all that surprised about this. He never looked to be the healthiest chap. He wasn't hugely overweight but he definitely led an unhealthy lifestyle, as this article makes clear.
Smith's friend and long-term collaborator, Griff Rhys Jones, spoke affectionately to the Telegraph in 2007 about Smith's hard-living lifestyle.
"Wherever you go into Mel's life you will hear terrible stories of people being 'Smithed'," Rhys Jones said. "Which is to say that they have been helplessly sucked into his hard-partying, up-all-hours lifestyle, only to be disgorged at some later stage in need of a rest cure. He has been like this all his life.
"He's very sociable, a tremendous late night man, a big eater, a drinker - although to be fair I have rarely seem him completely plastered - and there is a long and gratifying list of people who have failed to keep pace with him."
He fit the stereotype of the self-indulgent comic, that's for sure! Still, he had a very good run -- particularly compared to some other comics. And dying suddenly from a heart attack is certainly not the worst way to go.