I don't dispute your statement that people tend to ridicule the unfamiliar, but you have chosen two particularly poor examples--in fact, they serve to illustrate an alternate phenomenon, which is that people ridicule things that are known to be wrong or not make sense.
In your Bigfoot example, we know that we have observed pretty much the entire surface of the planet--to the extent that we have conducted nearly an exhaustive search for Bigfoot and not found it. Therefore, Bigfoot is very unlikely to exist. Saying that you've seen Bigfoot, therefore, makes about as much sense as saying that bananas are blue and pink, unless you not only state the claim but provide amazingly good evidence of it.
In your meat eating example, if you eat animals, the animals themselves were directly or indirectly (by eating other animals) eating plants. So you not only eat the animals but you cause more plants to be destroyed by raising sufficient animals to sustain yourself. So the whole reasoning is nonsensical and that is what I think people would react to as ridiculous, not the all-meat diet. (The Atkins diet is not far from that already, and that's no longer viewed as ridiculous, although it was by some initially.)
Why do we find these things ridiculous? Hard to say--maybe it's just a way of applying social pressure (since the pressure of reason has clearly already gone awry) to prevent the spread of nonsense.
A better illustration of your claim might involve, say, cutting your ice-cream with a knife instead of a spoon. Ridiculous! Except there's no particular reason why spoons are better than knives at serving ice cream; they each have their own advantages (knives penetrate hard ice cream better, spoons scoop up soft ice cream better, it's easier to get the ice cream to come off of the knife, but spoons are less likely to damage the container). Or, that almost all objects are nearly entirely empty space. Ridiculous! (But true.) Honestly, I have little idea why people find such things ridiculous, but I am pretty sure that historically prominent psychologists and philosophers didn't get the right answer if they speculated on it. It's really hard to know these things with confidence--very hard to do appropriate experiments--and there are such a huge number of mutually contradictory just-so stories you could tell that one should suspect any individual one of being wrong.
But I will note another situation where this sort of reaction makes more sense. Suppose I go to a formal dinner with the Queen of England while wearing a backwards baseball cap instead of a tie. Ridiculous! Of course, neither the tie nor the baseball cap have any direct functionality, in all likelihood. Rather, the tie is a signal: I understand the social situation and am acting accordingly. The cap is a different one: I'm either totally unaware of or snubbing your silly social rules. This sort of thing can be used for "profiling"--really, just figuring out whether someone is one of you or not. Quite a valuable thing to do for a social primate, you know. Cultural ridicule can help enforce easy identification of group membership. Is that why we do it, in some sense? I don't know, but it's quite suggestive, isn't it?
Does this have anything to do with the ridicule of things functionally important or informational, rather than cultural? I don't know. I'd be surprised if there isn't some link, but human behavior is full of surprises.
