Whiny, pale, and (possibly) intent on eliminating the entire human race to make way for fluffy bunnies, the Hollywood Vegetarian is a thinly-veiled Strawman who is desperate for a bacon sarnie or would really like meat "if they just tried it". In some works, vegetarians or vegans are also shown arrogantly forcing their meat eating family and friends to stop eating meat, often referring to them as murderers.
The vast majority of vegetarians, however, are not like this. Most have eaten meat before and choose not to continue doing so for multiple reasons. The more well-rounded ones are also respectful of what others eat, though it helps if that respect is returned.
This section is a pick 'n' mix of stereotypical traits — any number of them can apply.
Really stupid, often a Dumb Blonde, sometimes to the point that they'll happily eat chicken "because they're not animals, they're birds!"
Note on the latter two: Vegetarianism isn't automatically healthier. There's the phenomenon of "pudding vegetarians" who mostly eat sweets and processed food, which is rather one-sided and more unhealthy than what Joe Average eats.
Repressed and secretly desperate for meat.
Continually pushes views on others - will almost always tell people that they're a vegetarian.
May have a rose-tinted view of the animal kingdom.
Almost always has humanitarian motives for not eating meat ("killing animals is wrong"). Vegetarians and vegans in fiction are very rarely motivated by concern for their own health or for the enviroment in general. If they do, it's usually a secondary motive to protecting animals.
In an advert for Quorm (a type of meat-replacement protein) the vegetarian is portrayed as the classic whiner "it's-a-phase" stereotype. Which is really weird when you consider that the stuff is aimed at primarily at vegetarians.
In Everything Is Illuminated, the (obsessive) Collector wants a memento of the field of sunflowers. He walks over to them and, instead of picking a few petals or something, he picks up a harmless, living, grasshopper and seals it in one of his polypockets where, we are forced to assume, it suffocates. He is also incredibly pale.
In Malcolm in the Middle , we find out his class is full of these at the Krelboyne picnic. Hal becomes a hero to all the beleagured dads in attendence (who very obviously were not vegetarians) by sneaking in real meat, but chaos ensues when the Krelboynes realize their "tofu" is bleeding.
Regine from Living Single becomes one after watching a daytime talk show where its said being a vegetarian not only good for you, but will keep you active and young looking when you get old (as well as have a hot, young boyfriend).
CSI: Crime Scene Investigation averts this completely with Sara Sidle. She did complain when Grissom wanted her to clean up his meat experiment, but now they seem to have a good understanding, and she never fell into the stereotypes at all. She does occasionally point things out to someone in the break room, but doesn't get pushy about their choices.
Played straight with the paleontology student who died on the set of the animatronic Walking With Dinosaurs live show. He not only pressured his girlfriend into swearing off meat, but he idolized herbivorous dinosaurs and despised the carnivores to a degree that seems idiotic in a science student.
Similarly averted on Bones with Temperance Brennan. Her diet change was an outgrowth of one, the pig farm incident at the end of season one that was connected to her mother's death and two, writing in the real life choices of the actress playing her.
Gordon Ramsay at one time seemed to believe this of most vegetarians and has pulled stunts before, such as allowing a cook to tell a passing vegetarian that something with bacon in it was vegetarian. He also organized a TV stunt on his program The F Word where he filled a restaurant with (supposed) vegetarians who had all agreed to try meat (presumably to put pressure on any regular veggies watching). He has since softened his stance somewhat but is still almost rabid about eating meat, which he sometimes goes to quite bizarre lengths to push.
The Best Page In The Universe portrays all vegetarians everywhere like this, and urges carnivores to eat more meat than usual to compensate for the animals that aren't dying.
Averted by Karen in SBB Brothers' Sims Big Brother 8. While her interview makes her sound like one, where she states her goal is to introduce one houseguest to veganism or at least vegetarianism, but she doesn't outright say it.
Also Lisa once fell in with a crowd of environmental terrorists. She develops a crush on the leader and visits him in jail.
Lisa: I think your protest was incredibly brave. Jesse: Thank you. This planet needs every friend it can get. Lisa: Oh, the earth is the best! That's why I'm a vegetarian. Jesse: Heh. Well, that's a start. Lisa: Uh, well, I was thinking of going vegan. Jesse: I'm a level 5 vegan - I won't eat anything that casts a shadow.
Admittedly she's young and has a crush; there's no strong indication she doesn't believe veganism is a good thing. In fact Jesse could be the bigger Strawman, likely only being such a strong vegan to swagger about his environmental message, not because he believes so strongly in it.
In Danny Phantom, goth girl Sam tries to get meat banned from the school café in favour of her parody vegetarian diet of turf (yes turf, as in grass) - the result is mass protests from both sides. It's played for laughs, however, and is an Affectionate Parody (Her opposite, Tucker, is utterly meat-obsessed).
Hayley in American Dad!. An adamant vegetarian, yet in one episode, she spends her time stuck in Africa with Stan and Steve in the American Embassy, clearly eating meat dishes. In another, she admonishes her boyfriend Jeff for eating a corndog, telling him they're vegetarian. Jeff replies "Still?"
My Little Pony: Friendship is Magic: Despite the entire cast being canon vegetarians, having bodies hard-wired for eating plants, living in a society and ethical system informed by that diet, surrounded by other sapient species (cows) and semi-sapients (rabbits) and by monsters that want to eat them - you'd be surprised how often fanfics portray them as having a secret craving for bacon. Rainbow Dash in particular seems particularly prone to this.
This has gotten worse/better with Equestria Girls, where Twilight travels to a humanoid world. And since these "humans" are implied to eat meat, Twilight can't resist it anymore.... In a way the MLP: Fi M fanfic trend of portraying horses as rabid closet omnivores or even carnivores who simply cannot resist the lure of meat is so far beyond the ridiculous, and such an exercise in Meat Versus Veggies 'tribalism' as to be worthy of psychological study in of itself.