I have been a judge at my local herp society's show each year for a few years now. I can tell you outright that yes plenty of people seem to have what I would consider obese snakes. Other judges who know a lot more than me about the topic also agree that lots of the entrants are obese. We arrive at this decision first of all just b comparing the snake at which we are looking with other apparent healthy individuals of the same species. If the snake literally looks as if it is fat or plump, then it is possibly obese. A closer look will often reveal a good tell as to whether or not the snake is obese, and that is if you can see the skin between the scales on a straight portion of the snake (not on a curved part of its body) while the snake is lying there at rest. What that often means is that the snake is so overweight that it is stretched to the max much like a 5 pound sausage in a 3 pound skin; of course it could also be indicative of any disease that would bloat the snake. I don't know if all snakes would develop this condition but I have seen it in, to name a few, such snakes as Corn Snakes, Hog Nose Snakes, Sonoran Gopher Snakes, Kingsnakes, and as I recall some sort of python.
I have noticed that when a snake appears overweight and, when we tell the owner such may be the case, the keeper usually says something to the effect that: 'Oh no my cute little baby is just well fed and not overweight. I love it so much I would never feed it too much.' I definitely think that the lovey dovey attitude of the keeper toward the animal may have something to do with it. I am not saying that just those who have this attitude toward their snakes are all who ever feed them too much but; I am saying that obesity seems more prevalent among snakes kept by owners with that attitude. I guess it is sort of like mom wanting to make sure you ate everything on your plate even though your belly hurts. That is just what I have noticed in my limited experience with seemingly obese snakes.
Just for the record, my guess is that each year, using corn snakes as an example, 10 to 20 percent shown at our annual show are likely overweight. We of course try to discourage overfeeding, but lots of people don't seem to get it. I do not know for certain that a snake's being somewhat obese would greatly effect its health negatively but, it is possible that its life span could be shortened, or that breeding capabilities could be lessened. Then again, obesity in a female before the mating season may help her produce eggs; but I would tend to think that a healthy weight as opposed to being overweight would be best for any critter. I can say that obese snakes do seem more lethargic than those that appear to have a weight more proportionate to length.
As far as too skinny goes, if you can see loose skin folds on a snake easily at any point (this goes for most commonly kept species), and or if the snake looks skinny with the backbone almost looking as if it is protruding or just has skin draped over it, then the snake could be underweight. If the snake is seriously underweight the backbone thing is almost always evident, the snake may appear sunken in at the sides, and it will look like little more than skin draped overs its bones. This can be caused by being underweight due to too little food, or due to a parasite load, and can also be caused by dehydration.
A healthy snake, in my opinion, looks well filled out, not stretched out diameter wise. While there may be some places where the skin looks loose as the snake curves or coils, it is overall fairly taut looking in most species. The snake is strong and the muscular development is evident as opposed to being lacking in a too thin snake, or hidden under plumpness in a too thick snake.
If you want an idea of how your snake should look, go out and look at others of the same or similar species. After seeing a large number of them, you will develop a prett good picture of fat versus thin versus healthy weighted snakes.