Straw Vegetarian

Whiny, pale, and (possibly) intent on eliminating the entire human race to make way for fluffy bunnies, the Hollywood Vegetarian is a thinly-veiled Straw Man 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. Many 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.

Common Stereotypes:

This section is a pick 'n' mix of stereotypical traits -- any number of them can apply.

Really stupid, often a Dumb Blonde
Usually female
Very pale
Very weak
Repressed and secretly desperate for meat.
Snobby
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.
May even wish that humans were wiped out so they'd stop killing animals.
Pretty much a Fundamentalist.
Possibly a member of an Animal Wrongs Group.
Almost has to be a Strawman Political by definition (generally portrayed as annoyingly self-righteous even if they have a point)

Advertising

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.

Comic Books

Lance Blastoff encounters one in Frank Miller's Tales to Offend. She is immediately converted when she smells the delicous aroma of roast dinosaur Lance is cooking.

Film

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.

Live-Action TV

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.
Dick Solomon became one of these as a Compressed Vice on 3rd Rock from the Sun, reverting to his normal meat-eating ways at the end of the episode. Justified by the Rule of Funny and the fact that Dick's personality is just that extreme.
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 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.
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.

Newspaper Comics

Hector's girlfriend Autumn from Zits. The first time she saw Jeremy eating a burger, she got a hysterical fit as if she truly had never seen anything so terrible before.

Web Original

One of these features in this story from Bash.
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.

Western Animation

The Simpsons: Lisa tries to convince everyone not to eat meat at Homer's barbeque and eventually throws away the roasted pig. At the end of that episode, Lisa learns from Paul and Linda McCartney not to be judgemental about non-vegetarians, after which she mostly settles down—but it doesn't stop her occasionally lapsing into aggression in later episodes, mostly as jokes.
- Also Lisa once fell in with a crowd of environmental terrorists (on account of the leader was hot). She 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.

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).
In DC Super Hero Girls, Jessica is an aversion, being a realistic vegan. Her “friend” Pamela, however (who is secretly Poison Ivy) is the opposite, not eating any plant-based food at all (except sliced bread, once), as she views plants as more deserving of life than people. This actually makes her more of a hypocrite than most characters on this list (given that quite a lot of plant matter is used to raise cattle and process meat to make it fit for human consumption) and it is a very unhealthy diet, the likely reason Pamela is so frail and sick-looking in her civilian identity.