Anything can be sinful, conceivably.
What makes something sinful?
Oh, we know about stuff like, "Sin is missing the mark," but what is it that makes something miss the mark?
Basically, it comes down to this: Is anyone hurt?
If I hurt my neighbor, of course that's sinful, but what if I don't hurt my neighbor at all? What if I only hurt myself? Is that sinful?
Is eating a lot of hot dogs or pizza slices or meatballs or whatever going to hurt anyone else? Is it going to hurt me?
I could be mistaken, but it seems to me that you don't hear very many stories about people being hurt in such contests. They might overly stretch their stomachs, but the stomach soon returns to its former size.
Perhaps the eating is a sign of gluttony, but if it is, then the sin was already there, wasn't it?
Now here's another thought. Eating contests are fun. They also mock the hungry around the world. If something is fun, does that mean it must be sinful? I don't find that a convincing idea, but I know people who think if anything is fun, then its morality is automatically suspect.