• It went pear-shaped — started good but went bad
• Spud-head — ugly and stupid
• Nose like a cucumber — disfigured/warty/bumpy nose
• Stomach/arse like a sack of potatoes — huge arse
• Coconut head — stupid
• Strawberry nose — similar to cucumber nose but red
• Melon head — big head
• Pea-brain — dumb

As can be seen from the list, veggies and fruit are usually used to describe a body part, which makes sense as these objects rarely have personality assigned, because they're inanimate.

Calling someone bananas means that they're crazy.

Also (although it's not aimed at a human), a car which is of bad quality is often called a lemon.

In UK English:

• Cabbage is used as a (tasteless and insensitive) slang term for someone in a persistent vegetative state, so that would definitely be an insult

• plum and turnip are both used as mild disparagements

You are what you eat. That's why I don't eat nuts. I don't much care for Swedes either and Kiwis are there and thereabouts. I consume ugli fruit only in the dark and cumquats are best left unmentioned. Last and least, there's the humble pigface.

^{Under a different moniker, I might have also derided the wonderful betel nut.}

It's not often used as an epithet, but saying somebody is like a cabbage is (or, particularly, that a group, team, committee or cabinet is like a row of cabbages) is certainly known.

Eggplant has been used as a ethnic slur towards people with African heritage. http://en.wikipedia.org/wiki/List_of_ethnic_slurs_by_ethnicity

Actually, I believe the slur "shyster" is generally reserved for Jewish attorneys. I believe the proper slur for someone like myself would be "eggplant".

—Robert Clayton Dean (portrayed by Will Smith), Enemy of the State http://en.wikiquote.org/wiki/Enemy_of_the_State

A gooseberry is an additional person who is neither necessary nor wanted in a given situation, in particular a third person with two others who are engaged romantically (or would like to be).

http://en.wiktionary.org/wiki/gooseberry

To call something "peanuts" means it is insufficient. Like, "We can pay you $40,000." "What?! That's peanuts!" I've never heard a person called "peanuts" in this sense though, just a monetary amount or something else offerred as having value.

"He's a few fries short of a Happy Meal" means he is stupid. But it's not really the comparison to a potato that labels him as stupid, but rather the insufficient quantity of potatos.

Saying someone is "wooden" means he fails to show normal human emotions.

Others have mentioned bananas and couch potatos. Those are probably the best examples.

While this is an amusing exercise, I'm not sure what the point is. As an argument for vegetarianism, I could only say, So what? If plants are more "respectable" than animals, wouldn't that be a reason to see animals as more "disposable"?

Like a vegetarian friend of mine once said, "Oh, wait, you don't understand. I'm not a vegetarian because I love animals. I'm a vegetarian because I hate plants."

• full of beans — spouting nonsense, silly
• string bean — a tall, skinny person
• couch potato — a lazy person

I would love to get some native speakers of other languages on here to contribute vegetable insults from their mother tongues... even though it's not pertinent to English (but possibly is relevant to your FB argument).

Although not a vegetable (it's a grain) calling someone corny is a mild insult meaning they tell bad jokes (also being called cheesy which is a dairy product).

Another insulting fruit to add to the list is calling somebody a prune.