I'd say it's a mixed bag, leaning towards negative, but I have seen vegetarian characters who have to contend with (obviously meant to be) stupid and/or ignorant people who cannot understand the concept.
On the other hand, there is a definite association with vegetarianism and tree-huger or at least quirky types (off the top of my head, examples from two terrible - no offence - shows such as Friends and Dharma and Greg seem to bear this out).
I don't watch Simpsons much beyond the 6th season, but for some unknown reason I saw one the other day where Bart gets a bull and tries to rescue him. Besides the episode being free from laughs, I thought they painted the whole veg*n movement in the most stupid way imaginable. Not only are we led to believe that veg*ns are at least partially crazy, but we also get this idea that veg*ns don't get enough protein and have to take a million pills. I mean, can we lay off the stupid veg*ns don't get enough protein already??? It's the one thing that almost everyone gets more than enough of in this hemisphere.